The Collaborative International Dictionary of English v.0.48:
Homology \Ho*mol"o*gy\, n. [Gr. ? agreement. See Homologous.]
1. The quality of being homologous; correspondence; relation;
as, the homologyof similar polygons.
2. (Biol.) Correspondence or relation in type of structure in
contradistinction to similarity of function; as, the
relation in structure between the leg and arm of a man; or
that between the arm of a man, the fore leg of a horse,
the wing of a bird, and the fin of a fish, all these
organs being modifications of one type of structure.
Note: Homology indicates genetic relationship, and according
to Haeckel special homology should be defined in terms
of identity of embryonic origin. See Homotypy, and
3. (Chem.) 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.
General homology (Biol.), the higher relation which a
series of parts, or a single part, bears to the
fundamental or general type on which the group is
Serial homology (Biol.), representative or repetitive
relation in the segments of the same organism, -- as in
the lobster, where the parts follow each other in a
straight line or series. --Owen. See Homotypy.
Special homology (Biol.), the correspondence of a part or
organ with those of a different animal, as determined by
relative position and connection. --Owen.