Rainier vs Mt. Rainier

Rainier and Mt. Rainier both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rainier	Yes	No	No	No
Mt. Rainier	Yes	No	No	No

As nouns, Rainier and Mt. Rainier are synonyms defined as:

Rainier and Mt. Rainier: a mountain peak in central Washington; highest peak in the Cascade Range; (14,410 feet high)

Other synonyms of Rainier include Mount Rainier, Mount Tacoma.

Rainier (noun)	Mt. Rainier (noun)
a mountain peak in central Washington; highest peak in the Cascade Range; (14,410 feet high)	a mountain peak in central Washington; highest peak in the Cascade Range; (14,410 feet high)