ZIP ON A FIELD' The term 'ZIP ON A FIELD' is a three letter acronym which stands for 'Zone Improvement Plan'. This was a system developed in the United States in 1963 to help simplify the process of sorting and delivering mail. It assigns each address a unique code consisting of five digits, which are used to identify the geographic area and post office where the mail is to be delivered. The ZIP code system is still in use today and is an important part of the US postal service.