In the United States, a historic district is a group of buildings, properties, or sites that have been designated by one of several entities on different levels as historically or architecturally significant. Buildings, structures, objects and sites within a historic district are normally divided into two categories, contributing and non-contributing. Districts greatly vary in size: some have hundreds of structures, while others have just a few.
The first U.S. historic district was established in Charleston, South Carolina in 1931, predating the U.S. federal government designation by more than three decades.
More information on the Wikipedia page .
The Wikipedia category page, with a list of districts is