Diagram of the skulls of a Monitor lizard and a Crocodile: homologous bones have the same colours.

A homologoustrait is any characteristic which is derived by evolution from a common ancestor. This is contrasted to analogous traits: similarities between organisms that were evolved separately.

The term existed before 1859, but got its modern meaning after Darwin established the idea of common descent.^p45The pre-Darwinian naturalists Cuvier, Geoffroy and Richard Owen, also used the idea.