Top 100 Most Innovative Technologies of the 21st Century

Artificial Intelligence

This guide provides an introduction to the basics of Artificial Intelligence (AI). It covers the fundamentals of AI, including its history, current applications, and potential future developments. It also provides an overview of the different types of AI, such as machine learning, deep

Autonomous Vehicles

Autonomous vehicles are revolutionizing the way we travel. This technology is rapidly advancing, and soon we may be able to rely on self-driving cars to get us from point A to point B. This article explores the potential of autonomous vehicles, the challenges they face

Cloud Computing

Cloud Computing is a technology that allows users to access data and applications over the internet. It is a way of storing, managing, and processing data on remote servers hosted on the internet instead of on local servers or personal computers. Cloud Computing provides users with the ability to access their data and applications

Robotics

This course will explore the exciting possibilities of robotics and its potential applications in various fields. Students will learn about the fundamentals of robotics, including the basics of programming and engineering, and how to apply them to real-world problems. Through hands-on activities and projects

Internet of Things

The Internet of Things (IoT) is a network of physical objects, such as devices, vehicles, buildings, and other items embedded with electronics, software, sensors, and network connectivity, which enables these objects to collect and exchange data. IoT allows for the

5G Network

This article will explore the security challenges posed by the introduction of 5G networks and the measures that can be taken to ensure the safety and security of these networks. It will discuss the potential risks associated with 5G networks, such as increased vulnerability to cyber threats, and the need for improved

Blockchain

This book is designed to provide an introduction to the world of blockchain technology. It covers the basics of blockchain, including its history, how it works, and its potential applications. It also provides a comprehensive overview of the different types of blockchain, such as public, private, and

Augmented Reality

This guide is designed to help you understand the technology behind Augmented Reality (AR). We will explain the basics of AR, how it works, and how it can be used in various applications. We will also discuss the potential applications of AR and its implications

Virtual Reality

This guide is designed to provide an introduction to the exciting world of virtual reality. It will cover the basics of virtual reality, from the technology and hardware involved to the various applications and experiences available. It will also provide an overview of the current state

3D Printing

3D printing is a revolutionary technology that is quickly becoming the future of manufacturing. It allows for the creation of complex and intricate objects with a fraction of the time and cost of traditional manufacturing methods. With 3D printing, businesses can produce parts and products with greater accuracy

Big Data

This course provides an introduction to the field of Big Data. It covers the fundamentals of Big Data, including its history, the different types of data, and how to analyze and interpret it. It also covers the various tools and techniques used to manage and analyze Big Data, as

Quantum Computing

Quantum computing is a revolutionary technology that has the potential to revolutionize the way we process data and solve problems. This introduction to quantum computing will provide an overview of the fundamentals of quantum computing, including the basics of quantum mechanics, the principles of quantum computing, and the potential

Wearable Technology

This guide provides an overview of the latest advancements in wearable technology and how they can help you stay connected. We'll discuss the different types of wearable technology, the benefits they offer, and how to choose the right device for your needs. We'll also

Machine Learning

This course is designed to provide an introduction to the basics of machine learning for beginners. It will cover topics such as supervised and unsupervised learning, deep learning, neural networks, and more. Through lectures, hands-on exercises, and real-world examples, students will

Nanotechnology

Nanotechnology is a rapidly developing field of science and technology that involves the manipulation and control of matter at the nanoscale. This introduction will provide an overview of the fundamentals of nanotechnology, including its history, current applications, and future possibilities. It will also discuss the

Renewable Energy

This guide provides an overview of renewable energy and how it can be used to power homes, businesses, and communities. It covers the basics of renewable energy sources such as solar, wind, geothermal, and hydroelectric power, as

Biometrics

This course provides an introduction to biometrics, the science of using physical characteristics to identify individuals. It covers the fundamentals of biometrics, including the different types of biometric systems, how they work, and the advantages and disadvantages of using biometrics for identification

Drones

Drones are quickly becoming one of the most revolutionary pieces of technology in the modern world. From military applications to commercial use, drones are changing the way we interact with the world around us. This introduction to drones will explore the technology behind them, the

Smart Homes

Smart homes are the future of home automation. With the latest technology, you can control your home from anywhere in the world. From turning on lights and adjusting the thermostat to unlocking doors and controlling security systems, smart homes are making home automation easier and

Self-Driving Cars

Self-driving cars are the future of transportation. They offer a safe, efficient, and convenient way to get around. With self-driving cars, you can travel without the worry of traffic, parking, or other drivers. Self-driving cars use advanced technologies such

Speech Recognition

Speech Recognition is a technology that enables computers to understand spoken language and convert it into text. It is used in a variety of applications, such as voice-activated commands, automated customer service, and dictation software. Speech Recognition technology is becoming increasingly sophisticated, allowing for more

Natural Language Processing

Natural language processing (NLP) is a field of artificial intelligence that focuses on enabling computers to understand and process human language. NLP techniques are used to analyze text, extract meaning, and generate natural language responses. NLP is used in a variety of applications, including natural language understanding,

Facial Recognition

This article will explore the potential of facial recognition technology as an automated security solution. We will discuss the advantages and disadvantages of using facial recognition technology, as well as the potential implications of its widespread implementation. We will also look at some of the

Gene Editing

Gene editing is a technique used to make precise changes to the DNA of living organisms. It involves the use of special enzymes to cut and modify DNA strands, allowing scientists to alter the genetic makeup of an organism and potentially cure genetic diseases. Gene editing has the potential to revolutionize medicine and

Smartphones

Smartphones have become an essential part of our lives, and choosing the right device can be a daunting task. This guide will help you make an informed decision when selecting a smartphone. We will discuss the different types of smartphones available, the features they

Graphene

Graphene is a revolutionary material that has the potential to revolutionize the way we think about technology. It is a single layer of carbon atoms arranged in a hexagonal lattice, and it is the thinnest and strongest material known to man. Grap

3D Bioprinting

3D bioprinting is a revolutionary technology that uses 3D printing to create living tissue and organs. This technology has the potential to revolutionize medical treatments, allowing for the creation of custom-made organs and tissue that can be used to replace damaged or diseased tissue. It

Autonomous Robots

This guide provides an introduction to the world of autonomous robots and the technology behind them. It covers the basics of robotics and automation, from the fundamentals of programming and hardware design to the latest developments in artificial intelligence and machine learning. It also provides

Smart Cities

This guide provides an overview of the concept of smart cities and how to create a sustainable urban environment. It covers topics such as the use of technology to improve urban infrastructure, the development of green initiatives, and the implementation of energy-efficient solutions.

Autonomous Weapons

This paper will explore the potential implications of autonomous weapons on human conflict. It will examine the advantages and disadvantages of using such weapons, as well as the ethical considerations that must be taken into account. The paper will also discuss the potential for autonomous weapons to be

Autonomous Underwater Vehicles

This course will explore the use of Autonomous Underwater Vehicles (AUVs) to explore the depths of the ocean. We will discuss the design and operation of AUVs, including the sensors and actuators used to control them. We will also discuss the challenges of

Autonomous Aerial Vehicles

This guide provides an overview of the benefits and challenges associated with the use of autonomous aerial vehicles (AAVs). It covers topics such as the advantages of using AAVs, the potential risks of using AAVs, and the legal and regulatory

Autonomous Spacecraft

This course will explore the possibilities of autonomous spacecraft, from the design and construction of the spacecraft to the potential applications and implications of their use. We will discuss the current state of the technology, the challenges and opportunities of developing and using autonomous spacecraft, and the potential

Autonomous Submarines

This course will explore the potential of autonomous submarines to explore the depths of the ocean. We will discuss the various technologies and techniques used to build and operate these submarines, as well as the challenges and opportunities they present. We will also discuss the potential applications

Autonomous Surface Vehicles

Autonomous Surface Vehicles (ASVs) are a rapidly emerging technology that promises to revolutionize marine exploration. With the ability to operate independently of human control, ASVs are able to explore the depths of the ocean in ways never before possible. This article

Autonomous Air Taxis

Autonomous air taxis are the future of urban transportation. With the potential to revolutionize the way we move around cities, autonomous air taxis offer a safe, efficient, and cost-effective alternative to traditional ground-based transportation. Autonomous air taxis are powered by

Autonomous Delivery Robots

This guide provides an in-depth look at the future of logistics with autonomous delivery robots. It explores the potential of this technology to revolutionize the way goods are delivered, from the development of the robots themselves to the impact they could have on the

Autonomous Farm Equipment

This guide provides an overview of the current state of autonomous farm equipment and its potential to revolutionize the agricultural industry. It covers the different types of autonomous farm equipment, their advantages and disadvantages, and how they can be used to increase efficiency and reduce costs

Autonomous Warehouse Robots

This guide provides an overview of the latest technologies available to automate warehouses and streamline operations. It covers the basics of autonomous warehouse robots, including their capabilities, advantages, and potential applications. It also provides an overview of the different types of robots available,

Autonomous Security Robots

This new technology offers a revolutionary way to safeguard your home and business. Autonomous security robots are designed to detect and respond to potential threats, providing a layer of protection that is both reliable and cost-effective. These robots are equipped with advanced sensors and cameras

Autonomous Construction Robots

This guide provides an overview of the emerging technology of autonomous construction robots and their potential to revolutionize the construction industry. It examines the current state of the technology, its potential applications, and the implications for the future of building. It also explores the ethical

Autonomous Mining Robots

Autonomous Mining Robots are robotic machines designed to autonomously extract minerals and resources from the earth. These robots are equipped with advanced sensors and algorithms to detect and identify valuable resources, as well as the ability to navigate and maneuver in challenging terrain. They are capable of working in remote and hazardous

Autonomous Logistics Robots

This guide provides a comprehensive overview of the rapidly evolving field of autonomous logistics robots and their potential to revolutionize the delivery industry. It covers the latest advancements in technology, the advantages of using robots for delivery, and the challenges of integrating them into existing systems

Autonomous Manufacturing Robots

This guide provides an overview of the benefits of autonomous manufacturing robots and how they can be used to improve production processes. It will discuss the advantages of using robots in manufacturing, such as increased efficiency, reduced labor costs, and improved safety. It will also provide

Autonomous Medical Robots

Autonomous medical robots are revolutionizing the healthcare industry. These robots are designed to assist medical professionals in providing quality care to patients. They can perform a variety of tasks, such as taking vital signs, administering medication, and performing diagnostic tests. These robots are also able

Autonomous Retail Robots

This guide provides an overview of the emerging technology of autonomous retail robots and its potential to revolutionize the way we shop. It explores the advantages of using robots in retail settings, such as increased efficiency, improved customer service, and reduced labor costs. It

Autonomous Military Robots

This comprehensive guide provides an in-depth look at the rapidly advancing field of autonomous military robots. It covers the history and current state of the technology, the ethical implications of its use, and the potential applications of the technology in the future. It also examines the various types

Autonomous Search and Rescue Robots

This project focuses on the development of autonomous search and rescue robots that can be used in hazardous environments. The robots will be designed to navigate through difficult terrain, detect and identify victims, and provide assistance in rescue operations. The robots will be equipped with sensors, cameras, and other tools to help

Autonomous Surveillance Robots

This comprehensive guide provides an in-depth look at autonomous surveillance robots and their potential applications. It covers the basics of robotics and how they can be used for surveillance, as well as the different types of robots available and their features. It also discusses the advantages and disadvantages of

Autonomous Underwater Robots

This course will explore the fascinating world of autonomous underwater robots. Students will learn about the design and engineering of these robots, and how they can be used to explore the depths of the ocean. Topics covered will include the principles of robotics, navigation and control, and the

Autonomous Waste Management Robots

This project focuses on the development of autonomous waste management robots. These robots will be able to autonomously navigate through a given environment and collect waste materials. The robots will be equipped with sensors and AI algorithms to detect and identify different types of waste materials, and will be able to sort the waste

Autonomous Weather Monitoring Robots

This project focuses on the development of autonomous weather monitoring robots. These robots will be able to collect data about the environment, such as temperature, humidity, wind speed, and other meteorological variables. The robots will be equipped with sensors and cameras to detect and record the data. The data collected

Autonomous Firefighting Robots

This description is about Autonomous Firefighting Robots, which are robotic machines designed to fight fires autonomously. These robots are equipped with advanced sensors and cameras to detect fires, and are capable of navigating hazardous environments to reach the source of the fire. They are also equipped with fire extinguishing

Autonomous Exploration Robots

This guide provides an introduction to autonomous exploration robots and their use in exploring unknown environments. It covers topics such as the design and development of robots, the use of sensors and navigation systems, and the challenges of operating in unknown environments. It also provides practical

Autonomous Agricultural Robots

This guide provides an in-depth look at the emerging technology of autonomous agricultural robots and the potential they have to revolutionize the farming industry. It covers the history and development of the technology, the current state of the industry, and the potential applications of

Autonomous Security Systems

This guide provides an overview of the latest advancements in autonomous security systems and how they can help keep your home safe. From motion-activated cameras to automated door locks, you'll learn the basics of these systems and how they can be used to protect your

Autonomous Surveillance Systems

This guide provides an overview of autonomous surveillance systems, including their benefits and risks. It explains the technology behind these systems and how they are used in various settings. It also discusses the implications of using these systems, such as privacy concerns, legal

Autonomous Traffic Management Systems

Autonomous Traffic Management Systems are computer-controlled systems designed to manage the flow of traffic in urban areas. These systems use sensors, cameras, and other technologies to detect and respond to traffic conditions in real-time. They can be used to reduce congestion, improve safety, and optimize traffic flow

Autonomous Power Grid Systems

This guide provides an overview of the latest technology in autonomous power grid systems, allowing you to automate your home's energy supply. It covers the basics of how these systems work, the benefits they offer, and the steps you need to take

Autonomous Water Management Systems

This guide provides an overview of the latest technology in autonomous water management systems. It explains how these systems can help you automate your water usage, from monitoring water levels to controlling irrigation systems. It also covers the advantages of using these systems, such as

Autonomous Waste Management Systems

This description outlines the features and benefits of autonomous waste management systems. Autonomous waste management systems are designed to automate the collection, sorting, and disposal of waste. These systems use advanced sensors and robotics to detect and classify waste, and then transport it to the appropriate disposal site. Autonomous waste

Autonomous Weather Monitoring Systems

This description is for autonomous weather monitoring systems, which are designed to collect and analyze data about the environment without the need for human intervention. These systems can be used to monitor temperature, humidity, wind speed, precipitation, and other weather-related data. They can also be used to detect changes

Autonomous Firefighting Systems

This description describes the development and implementation of autonomous firefighting systems. Autonomous firefighting systems are designed to detect and respond to fires in a variety of environments, including residential, commercial, and industrial settings. These systems use advanced sensors and artificial intelligence to identify and respond to fires quickly and accurately

Autonomous Exploration Systems

This guide provides an overview of the latest developments in autonomous exploration systems, and explores how they can be used to navigate unknown environments. It covers the fundamentals of autonomous exploration, including the various types of systems available, their capabilities, and the challenges they face

Autonomous Agricultural Systems

This comprehensive guide provides a detailed overview of the various autonomous agricultural systems available today. It covers the basics of automation, from the basics of sensors and actuators to more advanced topics such as machine learning and artificial intelligence. It also provides a comprehensive overview

Autonomous Logistics Systems

This guide provides an overview of autonomous logistics systems and how they can be used to optimize your supply chain. It covers topics such as the benefits of using autonomous systems, the different types of systems available, and how to implement them. It also provides

Autonomous Manufacturing Systems

This guide provides an overview of autonomous manufacturing systems and their potential to revolutionize the manufacturing industry. It covers the basics of autonomous manufacturing, including the different types of systems, their advantages and disadvantages, and the challenges of implementation. It also provides an in

Autonomous Medical Systems

This guide provides an in-depth look at autonomous medical systems and their applications. It covers topics such as the development and implementation of these systems, their potential benefits and risks, and the ethical considerations that must be taken into account when utilizing them. It

Autonomous Retail Systems

This guide provides an in-depth look at the emerging technology of autonomous retail systems. It explores the potential of this technology to revolutionize the way we shop, from the convenience of ordering items online to the automation of checkout processes. It also examines the

Autonomous Military Systems

This comprehensive guide provides an in-depth look at the rapidly evolving field of autonomous military systems. It covers topics such as the history of autonomous systems, current and emerging technologies, ethical considerations, and the implications for the future of warfare. It also examines the potential benefits and

Autonomous Search and Rescue Systems

This description covers the development and implementation of autonomous search and rescue systems. Autonomous search and rescue systems are robotic systems designed to autonomously search for and rescue people in dangerous or hazardous environments. These systems are capable of navigating complex terrain, detecting and locating victims, and providing assistance to victims in

Autonomous Space Systems

This course will explore the potential of autonomous space systems, from the development of robotic spacecraft to the use of artificial intelligence for mission control. Students will learn about the various technologies involved in the design and operation of autonomous space systems, as well as the challenges and opportunities

Autonomous Underwater Systems

This course will explore the use of autonomous underwater systems to explore the depths of the ocean. Students will learn about the various technologies used to build and operate these systems, as well as the challenges associated with their operation. Topics will include navigation, communication, and data collection

Autonomous Air Traffic Control Systems

This description focuses on the development and implementation of autonomous air traffic control systems. Autonomous air traffic control systems are designed to help reduce the risk of human error in air traffic control operations. These systems use advanced artificial intelligence algorithms to monitor and manage air traffic, allowing for more efficient and safe air

Autonomous Air Traffic Management Systems

This description is for an autonomous air traffic management system, which is a system designed to automate the management of air traffic. This system is designed to provide a safe and efficient way to manage air traffic, by using advanced algorithms and automation to monitor and control air traffic. It is capable of monitoring

Autonomous Air Traffic Surveillance Systems

This description outlines the features and benefits of autonomous air traffic surveillance systems. Autonomous air traffic surveillance systems are designed to provide enhanced safety and efficiency for air traffic control operations. These systems use advanced algorithms to detect, track, and monitor aircraft in the air, providing real-time data to air

Autonomous Air Traffic Warning Systems

This description outlines the features and benefits of Autonomous Air Traffic Warning Systems. These systems are designed to provide pilots with real-time information about potential hazards in the air, such as other aircraft, weather, and terrain. The systems use advanced artificial intelligence algorithms to detect and identify potential threats,

Autonomous Air Traffic Collision Avoidance Systems

This description outlines the features and benefits of Autonomous Air Traffic Collision Avoidance Systems (AATCAS). AATCAS is a revolutionary technology that uses advanced sensors and algorithms to detect and avoid potential collisions between aircraft in the air. This system is designed to increase safety and reduce

Autonomous Air Traffic Flow Management Systems

This description focuses on the development and implementation of autonomous air traffic flow management systems. These systems are designed to improve the efficiency and safety of air traffic control by automating the process of managing air traffic flow. They use advanced algorithms to monitor and analyze air traffic data in real time, allowing air

Autonomous Air Traffic Route Optimization Systems

This description covers the development and implementation of autonomous air traffic route optimization systems. These systems are designed to improve the efficiency and safety of air traffic navigation by using advanced algorithms to automatically calculate the most efficient and safe routes for aircraft. The systems use real-time data from air traffic control systems,

Autonomous Air Traffic Conflict Resolution Systems

This description outlines the development and implementation of autonomous air traffic conflict resolution systems. These systems are designed to help reduce the risk of mid-air collisions by utilizing advanced artificial intelligence algorithms to detect and resolve potential conflicts in real-time. The systems are capable of detecting potential conflicts and then proposing solutions

Autonomous Air Traffic Weather Avoidance Systems

This description outlines the development and implementation of autonomous air traffic weather avoidance systems. These systems are designed to help aircraft navigate around hazardous weather conditions, such as thunderstorms, hail, and high winds. They use advanced algorithms to detect and predict weather patterns, allowing aircraft to avoid dangerous areas and maintain

Autonomous Air Traffic Flight Planning Systems

This description outlines the capabilities of autonomous air traffic flight planning systems. These systems are designed to provide automated flight planning and navigation services for aircraft, allowing them to fly safely and efficiently in the air traffic control system. The systems use advanced algorithms and data analysis to generate optimized flight plans that take into

Autonomous Air Traffic Flight Tracking Systems

This description outlines the development and implementation of autonomous air traffic flight tracking systems. These systems are designed to provide real-time tracking of aircraft in the air, allowing for more efficient air traffic control and improved safety. The systems use a combination of sensors, cameras, and other technologies to detect and

Autonomous Air Traffic Flight Scheduling Systems

This description covers the development and implementation of autonomous air traffic flight scheduling systems. These systems are designed to automate the process of creating flight schedules, enabling airports to maximize their efficiency and reduce the risk of delays or cancellations. The systems use advanced algorithms to analyze flight data and create optimized schedules that

Autonomous Air Traffic Flight Data Analysis Systems

This project will focus on the development of autonomous air traffic flight data analysis systems. These systems will be designed to collect, analyze, and interpret data from air traffic control systems in order to improve safety, efficiency, and accuracy of air traffic control operations. The systems will be able to identify potential

Autonomous Air Traffic Flight Performance Monitoring Systems

This description covers the use of autonomous air traffic flight performance monitoring systems. These systems are designed to monitor the performance of aircraft in the air, providing real-time data on the performance of the aircraft and its systems. The data collected can be used to identify potential problems, identify areas of improvement

Autonomous Air Traffic Flight Safety Systems

This description outlines the features and benefits of an autonomous air traffic flight safety system. This system utilizes advanced technologies such as artificial intelligence, machine learning, and computer vision to monitor air traffic and ensure safe and efficient flight operations. The system is designed to detect potential hazards, alert pilots, and take

Autonomous Air Traffic Flight Control Systems

This description covers the development and implementation of autonomous air traffic control systems for aircraft flight. It will discuss the advantages and challenges of using these systems, as well as the potential implications for the aviation industry. It will also explore the current state of the technology, and the potential for further development and

Autonomous Air Traffic Flight Management Systems

This description is about autonomous air traffic flight management systems. These systems use advanced algorithms and artificial intelligence to automate the process of managing air traffic. They are designed to reduce the workload of air traffic controllers and improve the efficiency of air traffic management. The systems are capable of monitoring and controlling aircraft in

Autonomous Air Traffic Flight Navigation Systems

This description outlines the features and benefits of autonomous air traffic flight navigation systems. These systems are designed to provide pilots with a comprehensive and accurate view of air traffic and weather conditions, allowing them to make informed decisions about their flight paths. Autonomous air traffic flight navigation systems use advanced algorithms to monitor

Autonomous Air Traffic Flight Path Planning Systems

This description outlines the features and benefits of autonomous air traffic flight path planning systems. These systems are designed to provide a safe, efficient, and cost-effective way to manage air traffic. They use advanced algorithms to analyze flight data and generate optimized flight paths that reduce fuel consumption, minimize delays,

Autonomous Air Traffic Flight Path Optimization Systems

This description outlines the capabilities of autonomous air traffic flight path optimization systems. These systems are designed to provide automated, real-time optimization of flight paths for aircraft in order to reduce fuel consumption, minimize delays, and maximize safety. The systems use advanced algorithms to analyze and adjust flight paths in order

Autonomous Air Traffic Flight Path Monitoring Systems

This description outlines the features and benefits of Autonomous Air Traffic Flight Path Monitoring Systems. These systems are designed to provide an automated, real-time monitoring of air traffic flight paths in order to ensure the safety of passengers and aircraft. The systems use advanced sensors and algorithms to detect and track aircraft

Autonomous Air Traffic Flight Path Control Systems

This description outlines the features and benefits of autonomous air traffic flight path control systems. These systems are designed to provide a safe and efficient way to manage air traffic in busy airspace. They use advanced algorithms and sensors to detect potential conflicts between aircraft and provide automated solutions to ensure safe navigation. The systems

Autonomous Air Traffic Flight Path Management Systems

This description outlines the features and benefits of Autonomous Air Traffic Flight Path Management Systems. These systems are designed to provide a safe and efficient way to manage air traffic by automating the process of flight path planning and control. The system is capable of monitoring the airspace, predicting potential conflicts, and

Autonomous Air Traffic Flight Path Navigation Systems

This description provides an overview of autonomous air traffic flight path navigation systems. These systems are designed to safely and efficiently guide aircraft through the airspace. They use advanced algorithms to predict and adjust for potential conflicts, as well as to optimize the flight path for the most efficient route. The systems are designed

Autonomous Air Traffic Flight Path Analysis Systems

This description describes an autonomous air traffic flight path analysis system. This system uses advanced algorithms and artificial intelligence to analyze flight paths and identify potential risks and hazards. It can also provide recommendations on how to optimize flight paths for improved safety and efficiency. The system is designed to be used by air traffic

Autonomous Air Traffic Flight Path Simulation Systems

This description outlines the capabilities of autonomous air traffic flight path simulation systems. These systems are designed to simulate the behavior of aircraft in a variety of flight scenarios, allowing for the development of more efficient and safe air traffic control systems. The simulations are based on real-world data and can be used