coupler vs turnbuckle

coupler and turnbuckle both are nouns.

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

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

coupler: a mechanical device that serves to connect the ends of adjacent objects
turnbuckle: an oblong metal coupling with a swivel at one end and an internal thread at the other into which a threaded rod can be screwed in order to form a unit that can be adjusted for length or tension

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

coupler (noun)	turnbuckle (noun)
a mechanical device that serves to connect the ends of adjacent objects	an oblong metal coupling with a swivel at one end and an internal thread at the other into which a threaded rod can be screwed in order to form a unit that can be adjusted for length or tension