Common features of idiom

Non-compositionality: The meaning of a collocation is not a straightforward composition of the meaning of its parts. For example, the meaning of kick the bucket no longer has anything to do with kicking buckets (Kick the bucket means to die) even if it once did (the phrase "kicking the bucket" originally referenced suicide by hanging, wherein the despondent person would stand on a bucket with the noose around his or her neck, and then "kick the bucket" to allow the noose to tighten). Others, like the common yet semantically strange "leave well enough alone" may be a mondegreen for "leave both well and ill alone"[1]. See also collocation restriction.
Non-substitutability: One cannot substitute a word in a collocation with a related word. For example, we cannot say kick the pail instead of kick the bucket although bucket and pail are synonyms.
Non-modifiability: One cannot modify a collocation or apply syntactic transformations. For example, John Nag kicked the green bucket or the bucket was kicked have nothing to do with dying.

It is likely that every human language has idioms, and very many of them; a typical English commercial idiom dictionary lists about 4,000. When a local dialect of a language contains many highly developed idioms it can be unintelligible to speakers of the parent language; a classic example is that of Cockney rhyming slang. But note that most examples of slang, jargon and catch phrases, while related to idioms, are not idioms in the sense discussed here. Also to be distinguished from idioms are proverbs, which take the form of statements such as, "He who hesitates is lost." Many idioms could be considered colloquialisms.

In Spanish, the word idioma (= lengua) means language, and this is often reflected in their Second language's English—using idiom to refer to language.

Idiom

An idiom is an expression (i.e., 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, some debate has recently arisen on this subject.

In the English language 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 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.

Idioms hence tend to confuse those not already familiar with them; students of a new language must learn its idiomatic expressions the way they learn its other vocabulary. In fact many natural language words have idiomatic origins, but have been sufficiently assimilated so that their figurative senses have been lost.