all-terrain bike vs mountain bike

all-terrain bike and mountain bike both are nouns.

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

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

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

Other synonyms of all-terrain bike include off-roader.

all-terrain bike (noun)	mountain 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