A National Historic Landmark (NHL) is a building, site, structure, object, or district with historical importance. The United States government marks them as NHLs only if they have high historical significance. Out of more than 80,000 places on the National Register of Historic Places only about 2,430 are NHLs.
A National Historic Landmark District (NHLD) is a historic district that is recognized as an NHL. It may include contributing properties that have buildings, structures, sites or objects, and it may include non-contributing properties.