coupling vs fluid drive

coupling and fluid drive both are nouns.

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

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

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

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

coupling (noun)	fluid drive (noun)
the act of pairing a male and female for reproductive purposes	an automotive power coupling
a mechanical device that serves to connect the ends of adjacent objects
a connection (like a clamp or vise) between two things so they move together