What is the term for traits that have lost their original function?

The term for traits that have lost their original function is vestigial structures. A vestigial structure is an inherited anatomical feature or attribute that has lost some or all of the function it performed in an ancestor.

Vestigial does not necessarily mean completely useless. A structure may retain a reduced function or acquire a new one while no longer serving its ancestral role. The human appendix, for example, is commonly discussed as vestigial because its ancestral digestive role was reduced, although research indicates that it can still have immune and microbiome-related functions. Snake hind-limb remnants and the pelvic bones of whales are other examples.

Vestigial structures are usually identified by comparing related organisms and tracing anatomical homology. They provide evidence of common ancestry because a reduced feature can persist even after its original advantage disappears. Atavistic structures are different: an atavism is the occasional reappearance of an ancestral trait, such as rare hind-limb development in a snake. Homologous structures share evolutionary origin, while analogous structures have similar functions but evolved independently.