coupler vs swivel

coupler and swivel both are nouns.

Word	Noun	Adjective	Verb	Adverb
coupler	Yes	No	No	No
swivel	Yes	No	Yes	No

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

coupler: a mechanical device that serves to connect the ends of adjacent objects
swivel: a coupling (as in a chain) that has one end that turns on a headed pin

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

coupler (noun)	swivel (noun)
a mechanical device that serves to connect the ends of adjacent objects	a coupling (as in a chain) that has one end that turns on a headed pin

coupler (verb)	swivel (verb)
	turn on a pivot