The definition of an external unit is a device that is connected to a computer, either through a wired or wireless connection, to expand its capabilities. This could include a variety of devices such as a printer, scanner, webcam, or even a joystick. These external units allow the user to interact with the computer in ways that would not be possible without them. For example, a printer allows the user to print documents, while a scanner allows the user to scan documents into the computer. A webcam allows the user to video chat with others, and a joystick allows the user to play video games. External units are an essential part of the computing experience, as they provide users with the ability to do more with their computers than would otherwise be possible.