USA vs District of Columbia

USA and District of Columbia both are nouns.

Word	Noun	Adjective	Verb	Adverb
USA	Yes	No	No	No
District of Columbia	Yes	No	No	No

USA (noun)	District of Columbia (noun)
the army of the United States of America; the agency that organizes and trains soldiers for land warfare	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