The adjective vestigial derives from the Latin word vestigium, meaning "footprint, trace." It's most often used in biology to describe something that either didn't finish developing or has become, through evolution, pretty much useless.

What does it mean if something is vestigial?

1 of a body part or organ : remaining in a form that is small or imperfectly developed and not able to function : being or having the form of a vestige (see vestige sense 2) a vestigial tail Kiwis lack an external tail, and their vestigial wings are entirely hidden beneath a curious plumage—shaggy, more like fur than ...

Are vestigial organs fully developed?

Like most other physical features, however functional, vestigial features in a given species may successively appear, develop, and persist or disappear at various stages within the life cycle of the organism, ranging from early embryonic development to late adulthood.