The Collaborative International Dictionary of English v.0.48:

zip 4 \zip + 4\ (z[i^]p" pl[u^]s f[=o]r`), n.
   a full nine-digit zip code. See zip code.
   [PJC]

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]

WordNet (r) 3.0 (2006):

ZIP code
    n 1: a code of letters and digits added to a postal address to
         aid in the sorting of mail [syn: ZIP code, ZIP,
         postcode, postal code]