coupler vs mechanical device

coupler and mechanical device both are nouns.

Word	Noun	Adjective	Verb	Adverb
coupler	Yes	No	No	No
mechanical device	Yes	No	No	No

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

coupler: a mechanical device that serves to connect the ends of adjacent objects
mechanical device: mechanism consisting of a device that works on mechanical principles

coupler (noun)	mechanical device (noun)
a mechanical device that serves to connect the ends of adjacent objects	mechanism consisting of a device that works on mechanical principles