Definition, from OED: homologous:

1. Having the same relation, proportion, relative position, etc.;

2. (Biol.) Having the same relation to an original or fundamental type; corresponding in type of structure (but not necessarily in function); said of parts or organs in different animals or plants, or of different parts or organs in the same animal or plant. (Distinguished from analogous.)

c. (Med.) Derived from or involving an organism or organisms of the same species; also, involving or containing antibodies or antigens that react specifically with one another, as when an antibody has been produced by injection of an antigen.