Umbilicus

The umbilicus, also called the navel, is essentially a scar representing the former attachment of the umbilical cord in the fetus. The colloquial term for navel is belly button. It can form a depression, a type of umbilicus commonly referred to as an innie, or a protrusion, referred to as an outie. The underlying abdominal muscle layer also presents a concavity; its thinness at this point contributes to a relative structural weakness that makes it susceptible to hernia.