A live USB is a distribution of an operating system (OS) on a portable device that plugs into or connects to the universal serial bus (USB) port on a computer. Such devices include not only storage devices like USB drives and external hard drives but also consumer devices like smartphones and media players.
Live USB is a type of LiveDistro, which refers to a distribution of any operating system that can boot and run from removable media. That capacity makes it possible for users to carry their operating system of choice for use on other computers. Most live USBs have been Linux-based, often used to run that OS on Windows systems without making any changes to the host computer. Windows To Go is the first Microsoft operating system, other than those for embedded systems, to boot and run from a USB device.
Live USB is sometimes referred to as an operating system on a stick. In fact, the device can function as a "computer on a stick" because the increasing storage capacities of USB devices mean that users can store not only the operating system but also, if desired, applications and files.