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.