What is a vestigial organ

In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function. Vestigiality refers to genetically determined structures or attributes that have apparently lost most or all of their ancestral function in a given species, but have been retained during the process of evolution.

This is an organ that served a purpose in the distant ancestor of an organism, but is not longer pertinent in the presently existing, recently evolved, organisms.