The correspondence or resemblance of substances belonging
   to the same type or series; a similarity of composition varying by a
   small, regular difference, and usually attended by a regular variation
   in physical properties; as, there is an homology between methane, CH4,
   ethane, C2H6, propane, C3H8, etc., all members of the paraffin series.
   In an extended sense, the term is applied to the relation between
   chemical elements of the same group; as, chlorine, bromine, and iodine
   are said to be in homology with each other. Cf. Heterology.