Source : Webster's Revised Unabridged Dictionary (1913)

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

   {The anharmonic function} or {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.