Navel

Multiple realities
(covers information from several alternate timelines)
Tasha Yar, polywater intoxication

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")

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