An idiom is a phrase whose meaning cannot be deduced from the literal definition, 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; for instance, "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).
In the English expression to kick the bucket, for example, a listener knowing only the meaning of kick and bucket would be unable to deduce the expression's actual meaning, which is to die. Although it can refer literally to the act of striking a specific bucket with a foot, native speakers rarely use it that way. It cannot be directly translated to other languages – for example, the same expression in Polish is kopnąć w kalendarz (to kick the calendar), with the calendar being as detached from its usual meaning as the bucket in the English phrase is. The same expression in Dutch is het loodje leggen (to lay the piece of lead), which is entirely different from the English expression, too. Other expressions include break a leg and fit as a fiddle.
Another kind of idiom is the use of a single word to have multiple meanings, sometimes at the same time, and sometimes one meaning to be discerned from context. This can be seen in the (mostly uninflected) English language in the common use of the same word for an activity, for those engaged in it, and sometimes for a verb.