Navel
Multiple realities
(covers information from several alternate timelines)
Tasha Yar, a Human, wearing clothes that reveal her navel
The navel, also known colloquially as the belly button, was a small opening on the abdomen of mammals where the umbilical cord had once been attached.
Some forms of clothing revealed one's navel. (TOS: "Shore Leave", TNG: "The Naked Now") It could also be decorated with a piercing. (ENT: "In A Mirror, Darkly")
Entering a sacred hall on Dulaine with a covered belly button was considered a crime. (LD: "The Least Dangerous Game")