1. An expression whose meaning is not predictable from the usual meanings of its constituent elements, as kick the bucket or hang one's head, or from the general grammatical rules of a language, as the table round for the round table . . . .
2. a language, dialect, or style of speaking peculiar to a people. . . .
4. the peculiar character or genius of a language.
5. a distinct style or character, in music, art, etc.: the idiom of Bach.
A set expression of two or more words that means something other than the literal meanings of its individual words. Adjective: idiomatic.
example: (Captain James T. Kirk and Spock in Star Trek IV: The Voyage Home, 1986)
Kirk: If we
play our cards right, we may be able to find out
when those whales are being released.
“People use idioms to make their language richer and more colorful . . . . Idioms are used often to replace a literal word or expression, and many times the idiom better describes the full nuance of meaning. Idioms and idiomatic expressions can be more precise than the literal words, often using fewer words but saying more. For example, the expression it runs in the family is shorter and more succinct than saying that a physical or personality trait 'is fairly common throughout one's extended family and over a number of generations.'"
An "idiom" is a word or phrase which means something different from what it says. It is usually a metaphor. Idioms are common phrases or terms whose meanings are not literal, but are figurative and only known through their common uses.
Oxford English Dictionary
3. A form of expression, grammatical construction, phrase, etc., used in a distinctive way in a particular language, dialect, or language variety; spec. a group of words established by usage as having a meaning not deducible from the meanings of the individual words.
5. A distinctive style or convention in music, art, architecture, writing, etc.; the characteristic mode of expression of a composer, artist, author, etc.