A belly button (navel) that outwardly protrudes from the belly of the character/person.

The inverse being an "innie" navel (where it appears to be sunken in a small pit on the belly).

Most often seen on pregnant women as the growing womb pushes on and out the navel from the inside, especially during middle to late stages of pregnancy. Can also be seen on males and non-pregnant females as well, generally associated with either non-pregnancy related belly expansion/inflation, or just how the belly button had ended up since the belly button (navel) is formed from the child side remains of the umbilical cord after birth and the cutting of said cord.
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