idiomatic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Peculiar to or characteristic of a given language.
• adjective Characterized by proficient use of idiomatic expressions.
• adjective Resembling or having the nature of an idiom.
• adjective Using many idioms.
• adjective Peculiar to or characteristic of the style or manner of a particular group or people.

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

• adjective Of or pertaining to, or conforming to, the mode of expression peculiar to a language
• adjective 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Pertaining or conforming to the mode of expression characteristic of a language.
• adjective Resembling or characteristic of an idiom.
• adjective Using many idioms.
• adjective 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective of or relating to or conforming to idiom