set phrase vs idiomatic expression

set phrase and idiomatic expression both are nouns.

Word	Noun	Adjective	Verb	Adverb
set phrase	Yes	No	No	No
idiomatic expression	Yes	No	No	No

As nouns, set phrase and idiomatic expression are synonyms defined as:

set phrase and idiomatic expression: an expression whose meanings cannot be inferred from the meanings of the words that make it up

Other synonyms of set phrase include idiom, phrasal idiom, phrase.

set phrase (noun)	idiomatic expression (noun)
an expression whose meanings cannot be inferred from the meanings of the words that make it up	an expression whose meanings cannot be inferred from the meanings of the words that make it up