What is the best definition of a vestigial structure?

A vestigial structure is defined as a biological feature that has lost most or all of its original function through the process of evolution. These structures are remnants of organs or parts that may have been useful in an organism's ancestors but have become reduced or obsolete in the current form of the species. Examples of vestigial structures include the human appendix, which is thought to have been involved in digesting cellulose in the diets of our herbivorous ancestors, and the pelvic bones in whales, which are remnants from terrestrial ancestors that walked on land.

This definition highlights that while these structures may still be present in an organism, they no longer serve the primary function they once did, reflecting the changes the species has undergone over time due to adaptations to their environment.