programming, operating system
(I/O) Communication between a computer and its users, its storage devices, other computers (via a network) or the outside world. The devices the computer uses to do this are called "peripherals". What actually counts as I/O depends on what level of detail you are considering, e.g. communication between processors would not be considered I/O when considering a multiprocessor as a single system.
Important aspects of I/O are throughput, latency, and whether the communications is synchronous or asynchronous (using some kind of buffer).