What does "set phrase" mean?

An established expression whose wording is subject to little or no variation, and which may or may not be idiomatic.

Set phrase

Common synonyms include: fixed phrase, idiom, boilerplate phrase, canned phrase, clause, collocation.

noun

noun ·Uncommon ·Advanced level

Noun

1
An established expression whose wording is subject to little or no variation, and which may or may not be idiomatic.
"Bally remarks in passing, as Hall does not, that the inversion in toujours est-il que is part of a set phrase and hence invariable."
2
an expression whose meanings cannot be inferred from the meanings of the words that make it up wordnet
3
An idiomatic expression in general.
"If it proves clearly unfeasible to make the audience laugh at a thin and far-fetched joke, it is always better to change the way the joke works . . . for instance, a pun based on the speaker's taking literally some set phrase or metaphor with a pun based on phonetic similarity."

"Bally remarks in passing, as Hall does not, that the inversion in toujours est-il que is part of a set phrase and hence invariable."

From set + phrase.