The Zone Improvement Plan (or ZIP) Code is a grouping of numbers assigned by the U.S. Postal Service to designate a local area or entity in order to speedily deliver and distribute mail. ZIP Codes most often refer to a street section, a collection of streets, a structure or building, or a group of post office boxes, but the numbers do not rigidly conform to boundaries of cities, counties, states, and other places.

Depending on the area, a ZIP Code includes 5, 7, 9, or 11 digits. In the most common codes—five-digit ZIP Codes—the first digits divide the country into ten large groups of states numbered from 0 in the Northeast to 9 in the far West. Each state is divided into geographic areas identified by the second and third digits of the ZIP Code. For example, New York and Pennsylvania have ZIP Codes starting with numbers between 090 and 199; Indiana, Kentucky, Michigan, and Ohio begin with numbers between 400 and 499. The fourth and fifth digits of a ZIP Code identify the local delivery area.