Final answer:
A vestigial structure is a once-functional biological feature that has lost significance in current organisms, indicating evolutionary change and pointing to a common ancestry.
Step-by-step explanation:
A useless or rudimentary body part that is thought to have been important in ancestral populations but no longer has a known function is termed a vestigial structure.
Vestigial structures are biological remnants that were once necessary in ancestral forms of an organism but are no longer used in the same capacity. Examples of such include the human appendix, tailbone, and the hind leg bones in whales. Over the course of evolution, structures that are not utilized may decrease in size or function due to lack of selection pressure to maintain them. Consequently, these vestigial structures can provide evidence for evolution by pointing to a common ancestry where the structure was once functional.
This exemplifies the concept of an evolutionary mismatch, where vestigial structures persist despite changes in the organism's environment and lifestyle which render them obsolete.