ZIP code also zip code (z?p)n. A series of digits designating a specific locality in the United States, appended to a postal address to expedite the sorting and delivery of mail.[Originally a trademark.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.zip code n the US equivalent of postcode [C20: from z(one) i(mprovement) p(lan)]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ZIP? code` Trademark. a system used in the U.S. to facilitate delivery of mail, consisting of a code of five or nine numbers printed directly after the address. [1963; Z(one)I(mprovement)P(rogram)] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.