\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.