from
The Collaborative International Dictionary of English v.0.48
zip code \zip" code`\ (z[i^]p"k[=o]d`), n. [zone improvement
plan.]
Any one of a set of numbers assigned by the United States
Postal Service to designate a particular postal delivery
area. One or more zip codes are assigned to each post office.
The numbers initially had five digits (the initial code),
indicating state and post office. Later, four more optional
digits were added (the expanded code) indicating the box
number or delivery route. The full set of nine digits is also
referred to as {zip + 4}. The code is appended in an address
to the line bearing the city and state names, as in
"Plainfield, NJ 07062". [Also spelled {ZIP code} and {Zip
code}.]
[PJC]