universal joint vs coupler

universal joint and coupler both are nouns.

Word	Noun	Adjective	Verb	Adverb
universal joint	Yes	No	No	No
coupler	Yes	No	No	No

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

universal joint: coupling that connects two rotating shafts allowing freedom of movement in all directions
coupler: a mechanical device that serves to connect the ends of adjacent objects

Other hypernyms of universal joint include coupling.

universal joint (noun)	coupler (noun)
coupling that connects two rotating shafts allowing freedom of movement in all directions	a mechanical device that serves to connect the ends of adjacent objects