Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
ZIP code (n.)
ZIP is a consciously created acronym from Zone Improvement Program, used to describe the codes of originally five and now nine digits that the U.S. Postal Service uses to identify postal zones. The first two digits identify the state, and the last three the post office in that state. The additional four digits following a hyphen can specify individual postal customers or substations.
