set phrase vs locution

set phrase and locution both are nouns.

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

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

set phrase: an expression whose meanings cannot be inferred from the meanings of the words that make it up
locution: a word or phrase that particular people use in particular situations

Other hypernyms of set phrase include expression, saying.

set phrase (noun)	locution (noun)
an expression whose meanings cannot be inferred from the meanings of the words that make it up	a word or phrase that particular people use in particular situations