couplet vs stanza

couplet and stanza both are nouns.

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

As nouns, stanza is a hypernym of couplet; that is, stanza is a word with a broader meaning than couplet:

couplet: a stanza consisting of two successive lines of verse; usually rhymed
stanza: a fixed number of lines of verse forming a unit of a poem

couplet (noun)	stanza (noun)
a stanza consisting of two successive lines of verse; usually rhymed	a fixed number of lines of verse forming a unit of a poem
two items of the same kind