The navel (clinically known as the umbilicus, colloquially known as the belly button) is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby. All placental mammals have a navel, and it is quite conspicuous in humans. In humans, the navel scar can appear as a depression (often referred to colloquially as an innie) or as a protrusion (outie). Although frequently separated into these two categories, navels actually vary quite widely among people in terms of size, shape, depth/length, and overall appearance. As navels are scars, and not defined by genetics, they can serve as a way of distinguishing between identical twins in the absence of other identifiable marks.