What is the abbreviation for ZIP?

As used in US postal (or ZIP) codes, "ZIP" stands for "Zone Improvement Plan." US postal addresses in large cities used to include a Postal Zone number of two digits, as in "New York 13, NY." To increase the ability to automate the mail distribution system, zones were expanded to address numerically the entire country in a 5-digit Zip Code. The old, 2-digit Zone, where it existed, became the last two digits of the ZIP Code, with the first three digits defining the larger region in which the place was located.