Vestigial structures are a structure in an organism that has lost all or at least most of its original function in the course of evolution. This might include things like the human appendix. Vestigial structures have little to no importance to an organism but might be leftover from an ancestor.
Some other examples of vestigial structures in humans are muscles of the ear, wisdom teeth, the tailbone, body hair, and the semilunar fold in the corner of the eye. In 1890 Charles Darwin defined many of these vestigial structures as human attributes that are no longer useful to humans.