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input/output (I/O) The passing of information into or out of the central processing unit of a computer system, or the part of the system primarily dedicated to this activity. An important function of most I/O equipment is the translation between the host processor's signals and the sounds, actions, or symbols that are understood or generated by people. In some cases it may be translation between two types of machine-readable signals, as when a bar-code scanner reads the data-encoded package and translates it into an ASCII code. See also I/O.