United States vs District of Columbia

United States and District of Columbia both are nouns.

Word	Noun	Adjective	Verb	Adverb
United States	Yes	No	No	No
District of Columbia	Yes	No	No	No

United States (noun)	District of Columbia (noun)
the executive and legislative and judicial branches of the federal government of the United States	the district occupied entirely by the city of Washington; chosen by George Washington as the site of the capital of the United States and created out of land ceded by Maryland and Virginia
North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776