It's "ZIP Code™", trademarked by the US Postal Service (this is mostly to prevent others from misusing the name). It is exactly equivalent to a postal code, and

It's "ZIP Code™", trademarked by the US Postal Service (this is mostly to prevent others from misusing the name). It is exactly equivalent to a postal code, and most US residents will know both terms, although they will always say "ZIP". ZIP is an acronym for "Zone Improvement Plan", but no one knows that and it's not meaningful if you do!