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In computing, input/output, or I/O, refers to the communication between a digital processing system (such as a computer or an embedded system), and the outside world – possibly a human, another processing system or a device.
Inputs are the signals or data received by the system, and outputs are the signals or data sent from it.
I/O devices are used by a person (or other system) to communicate with the digital processing system. For instance, keyboards and mice are considered input devices of a computer, while monitors and printers are considered output devices of a computer. Devices for communication between computers, such as modems and network cards, typically serve for both input and output.
Note that the description of a device as either input or output depends on how it is viewed. Mouses and keyboards take as input physical movement that the human user outputs and convert it into signals that a computer can understand. The output from these devices is input for the computer. Similarly, printers and monitors take as input signals that a computer outputs. They then convert these signals into representations that human users can see or read. (For a human user the process of reading or seeing these representations is receiving input.)