The Collaborative International Dictionary of English v.0.48:

Idiomatic \Id`i*o*mat"ic\, Idiomatical \Id`i*o*mat"ic*al\, a.
   [Gr. 'idiwmatiko`s.]
   1. Of or pertaining to, or conforming to, the mode of
      expression peculiar to a language; as, an idiomatic
      meaning; an idiomatic phrase. -- Id`i*o*mat"ic*al*ly,
      adv.
      [1913 Webster]

   2. 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.
      [PJC]

WordNet (r) 3.0 (2006):

idiomatic
    adj 1: of or relating to or conforming to idiom; "idiomatic
           English" [syn: idiomatic, idiomatical]