Objects or events that have common origins and similar structures, even though their surface properties or functions may differ. In biology, where it is a controversial concept, it refers to similarities among species that are derived from a common ancestor, and which can apply to a morphological structure, a chromosome, or an individual gene or DNA segment. The forelimbs of chickens, dogs, humans and whales are examples of homologies in that the skeletons of these appendages are constructed of bones arranged in the same pattern due to the fact they were inherited from a common ancestor with a similar arrangement. As in the case of the wings of bats and birds, structures can be partially homologous and partially analogous (i.e., the wings are homologous because of common descent from the forelimb of a reptilian ancestor, but the modifications for flying are different and have evolved independently and are thus analogous).
See Analogy (biology), Character, Cladistics, Comparative method, Convergent evolution, General system theory, Isomorphism, Metaphor, Serial homology (or homonomy)