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.

A little-known suburb outside Columbus, Ohio, is the most in-demand real estate market in the U.S. for the second year in a row, according to a recent Realtor.com study. Gahanna, Ohio, ranked as the ...