An iiom is a term or phrase whose meaning cannot be deduced from the literal definitions and the arrangement of its parts, but refers instead to a figurative meaning that is 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 over time. This collocation -- words commonly used in a group -- changes the definition of each of the words that exist. As an expression, the word-group becomes a team, so to speak. That is, the collocated words develop a specialized meaning as a whole and an idiom is born e.g. He really threw me a curve when on our first date he asked if I could pay for the dinner. Note, in some cultures, when a man and a woman are courting each other, the male is traditionally the one who takes up the bill or pays the bill; however, times change and in many modern societies, a lot of couples go Dutch (yet another idiom).