idiomatic

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Peculiar to or characteristic of a given language.
2. adj. Characterized by proficient use of idiomatic expressions: a foreigner who speaks idiomatic English.
3. adj. Resembling or having the nature of an idiom.
4. adj. Using many idioms.
5. adj. Peculiar to or characteristic of the style or manner of a particular group or people.

Century Dictionary and Cyclopedia

1. Peculiar to or characteristic of a certain language; pertaining to or exhibiting the particular cast of a language or its characteristic modes of expression.
2. Given to or marked by the use of idioms: as, an idiomatic writer.

Wiktionary

1. adj. Pertaining or conforming to the mode of expression characteristic of a language.
2. adj. Resembling or characteristic of an idiom.
3. adj. Using many idioms.
4. adj. music Parts or pieces which are written both within the natural physical limitations of the instrument and human body and, less so or less often, the styles of playing used on specific instruments.

GNU Webster's 1913

1. adj. Of or pertaining to, or conforming to, the mode of expression peculiar to a language
2. adj. Of or pertaining to, or of the nature of an idiom{3}; having a meaning that is peculiar to itself and not predictable from general rules.

WordNet 3.0

1. adj. of or relating to or conforming to idiom