Formal:
* Idiom: This is the most common and straightforward term.
* Phraseological unit: A more technical term used in linguistics and lexicography.
* Idomatic expression: This emphasizes the "expression" aspect of the idiom.
Informal:
* Saying: This is a broad term that can include proverbs, adages, and idioms.
* Turn of phrase: This refers to the unique and characteristic way an idiom is expressed.
* Cliché: While not all idioms are clichés, some are overused and considered clichés.
The best term to use will depend on the context and the level of formality required.