What is the term for a feature that lost its original function during evolution?

A feature that has lost its original function during evolution is called a vestigial structure. Vestigiality means that a genetically inherited structure, behavior, or biological process retains some or all of its ancestral form after losing much of its former role.

Vestigial structures are identified through comparison with related organisms and ancestors. Human tailbones are remnants of ancestral tails, and the muscles that move the external ear are reduced versions of structures that were more useful in other mammals. Snakes and whales retain reduced hindlimb elements inherited from four-limbed ancestors.

“Vestigial” does not necessarily mean completely useless. A structure may lose its primary ancestral function while retaining a minor role or being adapted to a new one. The human appendix, for example, was long treated as functionless but appears to have immune and gut-related functions, even though it no longer performs the digestive role associated with ancestral herbivores.

The term is different from atavism. An atavism is the occasional reappearance of an ancestral trait, such as a rare tail-like projection in a human, whereas a vestigial structure is a normal inherited feature present in a population.