Outer Mongolia vs Mongolian People's Republic

Outer Mongolia and Mongolian People's Republic both are nouns.

Word	Noun	Adjective	Verb	Adverb
Outer Mongolia	Yes	No	No	No
Mongolian People's Republic	Yes	No	No	No

As nouns, Outer Mongolia and Mongolian People's Republic are synonyms defined as:

Outer Mongolia and Mongolian People's Republic: a landlocked socialist republic in central Asia

Other synonyms of Outer Mongolia include Mongolia.

Outer Mongolia (noun)	Mongolian People's Republic (noun)
a landlocked socialist republic in central Asia	a landlocked socialist republic in central Asia