An Acronym for Input/Output.
This very broad term generally refers to a computer's act of communication with the outside world (which the machine's mass storage devices are also seen as part of), such as by reading characters from the keyboard or a file, writing a buffer to disk, displaying something on the screen, receiving or sending something via a NIC, etc pp.
It is easier to define this term by exclusion: any movement of data is I/O unless both source and destination lie in the same address space.
See also: BIOS
