The connection or relation of bodies which have partial
   identity of composition, but different characteristics and properties;
   the relation existing between derivatives of the same substance, or of
   the analogous members of different series; as, ethane, ethyl alcohol,
   acetic aldehyde, and acetic acid are in heterology with each other,
   though each in at the same time a member of a distinct homologous
   series. Cf. Homology.