rusticism vs set phrase

rusticism and set phrase both are nouns.

Word	Noun	Adjective	Verb	Adverb
rusticism	Yes	No	No	No
set phrase	Yes	No	No	No

As nouns, set phrase is a hypernym of rusticism; that is, set phrase is a word with a broader meaning than rusticism:

rusticism: a rural idiom or expression
set phrase: an expression whose meanings cannot be inferred from the meanings of the words that make it up

Other hypernyms of rusticism include idiom, idiomatic expression, phrasal idiom, phrase.

rusticism (noun)	set phrase (noun)
a rural idiom or expression	an expression whose meanings cannot be inferred from the meanings of the words that make it up