By “idiom” we shall understand any composite linguistic expression, the meaning of which cannot be compositionally derived from the meanings of its parts. Examples would be “kick the bucket” “die”, or “hit the sack” “go to bed”.
- Dik (1989), a pag.86

If a basic predicate consists of a combination of words, it will be an “idiom”: although it consists of different words, it cannot be semantically derived by productive rule.
- Dik (1989), a pag.54