ZIP Code Glossary

A working reference for the terms that come up when people work with U.S. ZIP code data. Each entry leads with a plain-English definition and then explains the mechanics, the history, and where the data actually comes from. Compiled and reviewed by the Zip Code Near Me Data Team.

ZIP code

A ZIP code is a 5-digit number assigned by the United States Postal Service that identifies a mail delivery route or destination. ZIP stands for Zone Improvement Plan.

ZIP Code Tabulation Area

A ZCTA, or ZIP Code Tabulation Area, is a geographic area created by the U.S. Census Bureau to approximate the boundaries of a USPS ZIP code. ZCTAs are the unit of geography used to publish ZIP-level demographic statistics.

ZIP+4

ZIP+4 is a 9-digit form of the U.S. ZIP code, made up of the standard 5-digit ZIP plus a 4-digit suffix that identifies a specific block face, building, or post office box cluster.

ZIP code boundaries

USPS draws ZIP codes around mail delivery routes, not geographic areas. There is no official USPS map of ZIP boundaries; the Census Bureau approximates them with ZIP Code Tabulation Areas (ZCTAs).

PO Box ZIP code

A PO Box ZIP code is a ZIP code assigned by USPS exclusively to a bank of post office boxes at one post office, with no street-delivery residents or businesses. Census-based data sets do not publish demographics for PO Box-only ZIPs because they have no residential population.

Unique ZIP code

A unique ZIP code is a 5-digit USPS ZIP assigned exclusively to one large recipient (typically a corporation, university, or government agency) for the exclusive routing of mail to that recipient's addresses.

Related references

Reviewed by Zip Code Near Me Data Team. Last verified June 12, 2026.