A fixed expression is a standard form of expression that has taken on a more specific meaning than the expression itself. It is different from a proverb in that it is used as a part of a sentence, and is the standard way of expressing a concept or idea.
- all of a sudden
- come into mind
- fall in line
- I can assure you
- so to speak
- surf the web
- Siamese twins or irreversible binomials like mix and match, wear and tear, and rock and roll
- trinomials (3-word fixed expressions); e.g. lights, camera, action, signed, sealed, and delivered, and lock, stock, and barrel.