mountain bike vs off-roader

mountain bike and off-roader both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain bike	Yes	No	No	No
off-roader	Yes	No	No	No

As nouns, mountain bike and off-roader are synonyms defined as:

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

Other synonyms of mountain bike include all-terrain bike.

mountain bike (noun)	off-roader (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