A postal code is a series of letters or digits or both, sometimes including spaces or punctuation, included in a postal address for the purpose of sorting mail. Postal codes are used by postal services to determine where a piece of mail is being sent to and to simplify the task of bringing post to its destination. There are generally two widely accepted versions of a postal code: a ZIP code and a ZIP + 4 code.

In the United States, a ZIP code is a five-digit number that identifies a certain postal delivery area. The first three digits of a ZIP code correspond to the central mail processing facility used to sort the mail, while the last two digits determine how the mail is sorted and then later delivered to post offices. ZIP + 4 codes, established in 1983, identify an even more specific location within a specific ZIP code. They can help to identify a specific group of apartments or city block and are frequently utilized in larger, more dense cities.

Postal codes are used in many countries throughout the world, and although they are usually assigned to geographical areas, special codes are sometimes assigned to individual addresses or to institutions that receive large volumes of mail. For example, the postal codes in the Netherlands are alphanumeric, consisting of four digits followed by a space and two letters. Adding the house number to the postcode will identify the address, making the street name and town name redundant.

Postal codes are important not only for delivering mail and packages but also as a social sciences tool, allowing academics to categorize geographical areas more easily and providing a simple way to analyze behavior and trends from a socioeconomic and psychological perspective. They are even used as part of verifying identity and ensuring that things like credit cards arent being stolen.