What term describes a trait inherited from an ancestor but reduced or unused in a modern species?

A vestigial structure is a trait inherited from an ancestor but reduced or unused in a modern species.

Vestigial features persist because evolution modifies existing inherited structures rather than redesigning organisms from scratch. A structure can become smaller or lose its original function when environmental conditions or lifestyles change. It may still have a minor function, so “vestigial” does not always mean completely useless.

The human coccyx, or tailbone, is commonly cited as a vestigial structure because it is a remnant of ancestral tails. The tiny hind-limb bones of modern whales are another example; they are inherited remnants associated with land-dwelling ancestors and can still help anchor reproductive muscles.

Vestigial structures are often confused with homologous structures. Homologous structures share evolutionary ancestry, even when they perform different functions. A vestigial structure is a special case in which an inherited feature has been reduced or repurposed.