swivel vs coupler

swivel and coupler both are nouns.

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

As nouns, coupler is a hypernym of swivel; that is, coupler is a word with a broader meaning than swivel:

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

Other hypernyms of swivel include coupling.

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

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