The familiar five-digit number was part of a nationwide effort to modernize the mail. Most Americans know their ZIP code by heart. They type it into online forms, scribble it on envelopes and memorize it as part of their address. But not everyone knows the original meaning of the term. Indeed, the “zip” in “ZIP Code” isn’t just a catchy word ― it’s an acronym. “The ‘zip’ in ‘ZIP code’ stands for ‘Zone Improvement Plan,’” said Esteban Touma , a cultural and linguistic expert at the language-