fixed phrase

Noun

set phrase (plural set phrases)

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.1951, Gordon M. Messing, “Structuralism and Literary Tradition”, in Language, volume 27, number 1, page 3

   Synonyms:

   phraseme, fixed expression, fixed term

2. 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.1992, Stanislaw Baranczak, "How to Translate Shakespeare's Humor?: (Reflections of a Polish Translator)", in the Performing Arts Journal, volume 14, number 3, page 83