Definition of "set phrase"

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

American courts sometimes use cherry in place of apple, but the latter fruit vastly predominates. The American version is that rare set phrase that is not so well set, variations on the phrase being more common than the main phrase itself.

2009, Bryan A. Garner, Garner on Language and Writing‎, page 525

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