GLOSSARY
ZIP Code
By Justin Winter · Updated August 28, 2026
The 5-digit number USPS assigns to a geographic delivery area, used to route mail to the right regional and local sorting facility.
ZIP stands for Zone Improvement Plan. The first digit groups a set of states into one of ten national areas, and the remaining digits narrow that down to a specific region, city, or delivery zone — the system automated mail sorting can route on without reading a full address.
A ZIP code alone does not confirm a specific address is deliverable; that finer-grained check is what address standardization and delivery point validation add on top of it.
See also
- ZIP+4 — The 4-digit extension USPS adds to a 5-digit ZIP code to identify a specific delivery segment — often a single building, floor, or even a large mail recipient.
- Address standardization — Rewriting an address into the exact form USPS uses — canonical abbreviations, directionals, unit designators, and ZIP+4.
- Delivery Point Validation — A USPS check that confirms an address is a real, individually deliverable delivery point — down to the apartment or suite.
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