mountain bike vs all-terrain bike

mountain bike and all-terrain bike both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain bike	Yes	No	No	No
all-terrain bike	Yes	No	No	No

As nouns, mountain bike and all-terrain bike are synonyms defined as:

mountain bike and all-terrain bike: a bicycle with a sturdy frame and fat tires; originally designed for riding in mountainous country

Other synonyms of mountain bike include off-roader.

mountain bike (noun)	all-terrain bike (noun)
a bicycle with a sturdy frame and fat tires; originally designed for riding in mountainous country	a bicycle with a sturdy frame and fat tires; originally designed for riding in mountainous country