Vestigiality

In evolutionary biology and comparative anatomy, vestigiality is a term which describes homologous characters of organisms such as anatomical structures (vestigial structures), behaviors and biochemical pathways which have lost all or most of their original function in a species through evolution. They are typically in a degenerate, atrophied, or rudimentary condition. Vestigial structures are often called vestigial organs, although not all of them are actually organs.

Although structures usually called "vestigial" are largely or entirely functionless, a vestigial structure may retain lesser functions or develop new ones. Thus, a "vestigial wing" is one useless for flight, but may serve some other purpose.