An idiom is a term or phrase whose meaning can't be deduced from the literal definitions and the arrangement of its parts, but refers instead to a figurative meaning that's known only through common use. In linguistics, idioms are widely assumed to be figures of speech that contradict the principle of compositionality; however, this has shown to be a subject of debate. It may be better to refer to idioms as John Saeed does: words collocated together happen to become fossilized, becoming fixed o… (More on Idiom)