set phrase
Synonym discussion
Sense: noun/1
A set phrase, an idiom, and a collocation all describe words that travel together, but they differ in how fixed and how transparent they are. A set phrase has a locked form — "how do you do" or "by and large" — and you cannot change the words, though the meaning may still be clear. An idiom is a kind of set phrase whose meaning cannot be guessed from the separate words: "kick the bucket" has nothing to do with buckets. A collocation is looser; "heavy rain" and "strong coffee" pair naturally, yet you can still adjust them ("very heavy rain"). So Rachid would teach "spill the beans" as an idiom, "make a decision" as a set phrase, and "fast food" as a collocation.
Etymology
The term combines "set", meaning fixed or unchanging, with "phrase", a small group of words. "Set" comes from Old English settan, to cause to sit or place firmly, which gave the later sense of something put in a fixed position. "Phrase" arrived through Latin from Greek phrasis, meaning a way of speaking. Together they name wording that has been fixed in place.