dyad vs couplet

dyad and couplet both are nouns.

Word	Noun	Adjective	Verb	Adverb
dyad	Yes	No	No	No
couplet	Yes	No	No	No

As nouns, dyad and couplet are synonyms defined as:

Other synonyms of dyad include brace, couple, distich, duad, duet, duo, pair, span, twain, twosome, yoke.

dyad (noun)	couplet (noun)
two items of the same kind	a stanza consisting of two successive lines of verse; usually rhymed
	two items of the same kind