IDIOM, also (archaic) idiotism.
1. The SPEECH proper to, or typical of, a people or place; a DIALECT or local LANGUAGE: classics in the Tuscan idiom.

2. An expression unique to a language, especially one whose sense is not predictable from the meanings and arrangement of its elements, such as kick the bucket a slang term meaning ‘to die’, which has nothing obviously to do with kicking or buckets. In linguistics, the term idiomaticity refers to the nature of idioms and the degree to which a usage can be regarded as idiomatic. Some expressions are more holophrastic and unanalysable than others: for example, to take steps is literal and non-idiomatic in The baby took her first steps, is figurative, grammatically open, and semi-idiomatic in They took some steps to put the matter right, and is fully idiomatic and grammatically closed in She took steps to see that was done. These examples demonstrate a continuum of meaning and use that is true for many usages. No such continuum exists, however, between He kicked the bucket out of the way and He kicked the bucket last night (meaning ‘He died last night’). Such idioms are particularly rigid: for example, they cannot usually be passivized (no *The bucket was kicked) or otherwise adapted (no *bucket-kicking as a synonym for death).

Creative adaptations

Although idioms are normally simply slotted into speech and WRITING, they are occasionally subject to creative wordplay. The phrase on the other hand, a convention in the presentation of contrasting information (on the one hand, … on the other hand), is radically adapted in the following statement, from an article about animals:
A female needs an area which will provide enough food and denning sites for raising kittens, even in a year when food is in short supply. On the other paw, a male has a very much larger home range, which usually overlaps with those of several females (‘Just like Lions’, BBC Wild Life, Jan. 1989).

Spliced idioms

It is not unusual for even fluent speakers in the heat of conversation to blend or splice two idioms or COLLOCATIONS whose forms and meanings are similar. The following specimens were collected in the late 1980s: ‘That seems an interesting step to go down’ (splicing step to take and road to go down); ‘He stuck his ground’ (splicing stuck to his guns and stood his ground); ‘Language plays a decisive factor here’ (blending is a decisive factor and plays a decisive role).

See CATCH-PHRASE, CATCHWORD, CLICHÉ, FIXED PHRASE, HACKNEYED, HOLOPHRASE.

id·i·om / ˈidēəm/ • n. 1. a group of words established by usage as having a meaning not deducible from those of the individual words (e.g., rain cats and dogs, see the light). ∎  a form of expression natural to a language, person, or group of people: he had a feeling for phrase and idiom. ∎  the dialect of a people or part of a country.2. a characteristic mode of expression in music or art: they were both working in a neo-Impressionist idiom.

idiom proper language of a people or country, dialect; specific character of a language XVI; expression peculiar to a language XVII. — F. idiome or late L. idiōma — Gr. idiōma property, peculiar phraseology, f. idioûsthai make one's own, f. ídios own, private.
So idiomatic XVIII.