coupler vs fluid drive

coupler and fluid drive both are nouns.

Word	Noun	Adjective	Verb	Adverb
coupler	Yes	No	No	No
fluid drive	Yes	No	No	No

As nouns, fluid drive is a hyponym of coupler; that is, fluid drive is a word with a more specific, narrower meaning than coupler:

coupler: a mechanical device that serves to connect the ends of adjacent objects
fluid drive: an automotive power coupling

Other hyponyms of coupler include clevis, clutch, swivel, turnbuckle, universal, universal joint.

coupler (noun)	fluid drive (noun)
a mechanical device that serves to connect the ends of adjacent objects	an automotive power coupling