Vestigial describes a body part or feature that has lost most or all of its original function through evolution. Your appendix is a vestigial organ: it once helped distant ancestors digest tough plant material, but now it just sits there doing almost nothing (except occasionally getting infected). Whales have tiny vestigial hip bones buried deep in their bodies, leftover from millions of years ago when their ancestors walked on land with four legs.

These body parts are like footprints left behind by evolution, showing where a species has been. Ostriches have vestigial wings too small for flight. Humans have vestigial muscles that once moved our ears like a cat's, though most people can barely wiggle them now.

Sometimes people use vestigial more broadly to describe anything that once served a purpose but doesn't anymore. An old law that nobody follows might be called vestigial, or traditions that continue even though their original purpose has disappeared. The word reminds us that both living things and human institutions carry traces of their past, even when those traces no longer serve their original purpose.