%anharmonic%

a. [ F. anharmonique, fr. Gr. 'an priv. + &unr_; 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 ]