An idiom is a multiword construction that
- is a semantic unit whose meaning cannot be deduced from the meanings of its constituents, and
- has a non-productive syntactic structure.
An idiom often shows the following characteristics:
It is syntactically anomalous. It has an unusual grammatical structure.
- by and large
It contains unique, fossilized items.
- to and fro fro < from = away (Scottish)
- cobweb cob < cop = spider (Middle English)
Idioms contrast with the following:
Metaphors satisfy the first requirement for an idiom, that their meaning be obscure, but not the second, that they not be productive.
- throw in the towel
- throw in the sponge
Collocates may have restricted lexical possibilities or use archaic vocabulary such that they are not productive, but their meaning is not opaque.
- heavy drinking
- mete out