Input/outputinterfaces that different functional units of a system use to communicate among each other, or to the signals sent through those interfaces. Inputs are the signals received by the unit, and outputs are the signals sent from it. The term can also be used as part of an action; to "do I/O" is to perform an input or output operation.
The most common use of the term is for computer I/O devices that are used by a person (or other system) to communicate with a computer. For instance, keyboards and mice are considered input devices (from a human perspective) and monitors and printers are considered output devices (again, from a human perspective).
Notice however, that all the previous designations of these devices as either input or output change when the perspective changes. Mice and keyboards take physical movement from the human user as input and convert it into signals that a computer can understand as output. The output from these devices is treated as input by the computer. Similarly, printers and monitors take signals that a computer outputs as input. They then convert these signals into output representations that human users can see or read. For a human user the process of reading or seeing these representations is input. In data processing the conversion of information provided by one system (the user) into information or data useable by another system (the computer) is an example of what occures in a data processing system.
Strictly speaking, however, the combination of the CPU and main memory (i.e. memory that the CPU can read and write to directly, with individual instructions) is considered the heart of a computer, and any movement of information from or to that complex, for example to or from a disk drive, is also considered I/O.