The Free On-line Dictionary of Computing (30 December 2018):

input/output

    (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).

   (2003-12-04)