Vestigial organs were parts of one's anatomy that were retained despite an apparent lack or loss of function. The Human appendix was one such example.
In 2268, Doctor Leonard McCoy noted that the mute empath Gem didn't even have any vestigial vocal cords. (TOS: "The Empath")
In 2372, after crossing the warp 10 threshold, Tom Paris's vestigial organs atrophied and dissolved. (VOY: "Threshold")