Stands for input/output. This refers to the signals that come into and depart from the computing systemThis is a very generic term for any collection of components that, all together, can do something. Systems can be built from subsystems. Examples are your cell phone; your computer; the radio in your car; anything that seems like a "whole." (as opposed to the signals running internal to the computing system).
Inside the Internet of Things - Bryon Moyer Blog