What is the term for a body part that has lost its original function through evolution?

A vestigial organ is a body part that has lost all or most of its original ancestral function through evolution. It remains because evolutionary change modifies inherited structures rather than rebuilding an organism from scratch.

Vestigial does not always mean completely useless. A structure may retain a smaller function, serve a new purpose, or vary in its importance among species. The human appendix, for example, was long described as functionless but appears to have roles involving immune tissue and gut microbes. The key point is its reduced relationship to ancestral digestive functions, not absolute uselessness.

Other examples include the hind-limb remnants of whales and some snakes, reduced wings in flightless birds, and human tailbone structures inherited from tailed ancestors. Pseudogenes—DNA sequences that have lost their original protein-coding ability—can be molecular examples of vestigiality.

Vestigial organs differ from homologous structures, which share an evolutionary origin even when they perform different functions, and analogous structures, which perform similar functions but evolved independently. An atavism is the occasional reappearance of an ancestral trait, such as a rare human tail, rather than a permanently retained reduced structure.