Vestigial describes an organ or body part that continues to exist without retaining its original function, such as our appendix.
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. An ostrich's wings are vestigial because it's unable to fly — or do much of anything — with them. A penguin's wings, on the other hand, are not vestigial because it has found another use for them — to help it swim.