Input/output, or I/O, refers to the
interfaces 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 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 and
monitors and
printers are considered output devices. Notice however that all the previous devices have both input and output, but the perspective is from the computer. Mice and keyboards take physical movement as input and convert it into signals that a computer can understand, whereas printers and monitors take signals that a computer can output and convert them into representations that humans can see or read.