The term you’re looking for is vestigial structures. These are anatomical features that no longer serve a significant purpose in the current form of an organism, having been inherited from ancestors where they did serve an important function.
For example, in humans, the appendix is often cited as a vestigial structure. While it may play a role in the immune system, it is not essential for survival and many people live healthy lives after having it removed. Other examples include the pelvic bones in whales and the wings of flightless birds, which are remnants from their ancestors. These structures provide insight into an organism’s evolutionary history and the processes that have shaped its development over millions of years.