The interesting orifice in your belly.
It may stick outwards (an outie) or inwards (an innie).
Useful for catching spooge but tends also to collect nut butter and lint.
I had to take my tongue out of his belly button, because his treasure trail was tickling my ear.
The thing on one's stomach, which, if it is a hole, is good for holding tiny objects (candy, rocks, earbuds, etc.). Belly buttons are fun to play with...everyone has one, and everyone wants one.
Did you find that candy in your belly button?
Nah, man. Mine isn't deep enough.