The Collaborative International Dictionary of English v.0.48:

Anharmonic \An`har*mon"ic\, a. [F. anharmonique, fr. Gr. 'an
   priv. + ? harmonic.] (Math.)
   Not harmonic.
   [1913 Webster]

   The anharmonic function or anharmonic ratio of four
      points abcd on a straight line is the quantity
      (ac/ad):(bc/bd), where the segments are to be regarded as
      plus or minus, according to the order of the letters.
      [1913 Webster]