That which is corresponding in structure or origin, as in part of one animal to a similar part in another animal: The foreleg of a horse and the wing of a bird are homologs.

In genetics, it has the specific meaning of sequence identity(origin) and conservation of function(structure). Unfortunately, different people may divide up the genes slightly differently, so it can cause arguments.

This is especially problematic in the rare occasions where convergent evolution occurs. Since the structure is the same, but the origin is not, it could be argued that they are analogs instead.