A district possesses a significant concentration, linkage or continuity of sites, buildings, structures or objects united historically or aesthetically by plan or physical development.

A district derives its importance from being a unified entity, even though it is often composed of a wide variety of resources. The identity of a district results from the interrelationship of its resources, which can convey a visual sense of the overall historic environment or be an arrangement of historically or functionally related properties. For example, a district can reflect one principal activity, such as a mill or a ranch, or it can encompass several interrelated activities, such as an area that includes industrial, residential or commercial buildings, sites, structures or objects. A district can also be a grouping of archeological sites related primarily by their common components; these types of districts often will not visually represent a specific historic environment.

Please note the term "district" is often used as a generic term, and not all districts imply listing in the National Register of Historic Places. Please note there is no state district designation. Local districts may be designated using criteria set up through local planning and zoning ordinances. To be listed in the National Register, a district must go through the National Register listing process.

The Fair Park Texas Centennial Buildings in Dallas comprise a historic district that has been designated a National Historic Landmark.

Examples of districts include:
canal systems
commercial districts
college campuses
estates and farms with large acreage/numerous properties
groups of habitation sites
industrial complexes
irrigation systems
residential neighborhoods
rural historic districts
rural villages
transportation networks

Adapted from: Guidelines for Local Surveys: A Basis for Preservation Planning (NR Bulletin #24)