Merriam-Webster Collegiate Dictionary - homology

 noun  (plural -gies)  Date: circa 1656  1. a similarity often attributable to common origin  2.  a. likeness in structure between parts of different organisms (as the wing of a bat and the human arm) due to evolutionary differentiation from a corresponding part in a common ancestor — compare analogy  b. correspondence in structure between a series of parts (as vertebrae) in the same individual  3. similarity of nucleotide or amino acid sequence (as in nucleic acids or proteins)  4. a branch of the theory of topology concerned with partitioning space into geometric components (as points, lines, and triangles) and with the study of the number and interrelationships of these components especially by the use of group theory — called also ~ theory — compare co~