ZIP Code

ZIP Code, acronym for Zone Improvement Plan, a 5-digit code implemented (1963) to speed sorting and delivery of domestic mail. An optional extra 4 digits were added in 1981. The first number of the code identifies 1 of 10 large geographical areas in the United States where the mail will be delivered. The second 2 numbers indicate the metropolitan area, and the last 2 numbers represent the addressee's local post office. The ZIP code facilitates mechanical mail processing.