idiom

n. expression whose meaning cannot be derived from the combined meanings of it's individual elements; dialect, vernacular; characteristic style

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.
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Noun
1. a manner of speaking that is natural to native speakers of a language
(synonym) parlance
(hypernym) formulation, expression
2. the usage or vocabulary that is characteristic of a specific group of people; "the immigrants spoke an odd dialect of English"; "he has a strong German accent"
(synonym) dialect, accent
(hypernym) non-standard speech
(hyponym) eye dialect
(class) euphonious
3. the style of a particular artist or school or movement; "an imaginative orchestral idiom"
(synonym) artistic style
(hypernym) manner, mode, style, way, fashion
(hyponym) baroque, baroqueness
4. an expression whose meanings cannot be inferred from the meanings of the words that make it up
(synonym) idiomatic expression, phrasal idiom, set phrase, phrase
(hypernym) saying, expression, locution
(hyponym) ruralism, rusticism
(class) out of whack

n. idiom, dialect, characteristic style, form of a language spoken in a certain area or district

(n.)
The syntactical or structural form peculiar to any language; the genius or cast of a language.
  

(n.)
Dialect; a variant form of a language.
  

(n.)
An expression conforming or appropriate to the peculiar structural form of a language; in extend use, an expression sanctioned by usage, having a sense peculiar to itself and not agreeing with the logical sense of its structural form; also, the phrase forms peculiar to a particular author.