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Wikipedia
The navel, also called a belly button, umbilicus is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby.[1] All placental mammals have a navel. While it is fairly conspicuous in humans, in most mammals it appears only as a thin hairless line.
In humans, the scar can appear as a depression (sometimes colloquially referred to as an "innie") or as a protrusion ("outie"). Although they can easily be separated into 'innie' and 'outie' categories, navels vary quite drastically, in terms of size, shape, depth, and overall look, between people. As navels are essentially scars, and not in any way defined by genetics, they are often an easy way to distinguish between identical twins.