The navel (also called an umbilicus or, colloquially, 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. 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 (often referred to as an innie) or as a protrusion (outie). Although they can be separated into these two categories, navels actually vary quite drastically among people in terms of size, shape, depth/length, and overall appearance. As navels are essentially scars, and not in any way defined by genetics, they can serve as a way of distinguishing between identical twins in the absence of other identifiable marks.

Human anatomy

This section focuses on externally-visible aesthetics of the navel in human anatomy. Information regarding fetal circulation -- how oxygenated blood and nutrients are absorbed by a fetus from the umbilical cord -- may be found under umbilical vein and umbilical artery, both of which are umbilical vessels.

The umbilicus is an important landmark on the abdomen since its position is relatively consistent among humans. The skin around the waist at the level of the umbilicus is supported by the tenth thoracic spinal nerve (T10 dermatome). The umbilicus itself lies at the level between L3/L4 vertebrae.

The reason for the occurrence of an outie is extra skin left from the umbilical cord or umbilical hernias, although a child with an umbilical hernia will not necessarily develop an outie. As well as the visible depression on a person's abdomen, the underlying abdominal-muscle layers also present a concavity; thinness at this point contributes to a relative structural weakness, making it susceptible to hernia. During pregnancy, the uterus presses the navel of the pregnant woman outward. It usually retracts after birth.

The umbilicus is also used to visually separate the abdomen into quadrants. The navel comes in the center of the circle enclosing the spread-eagle figure in Leonardo da Vinci's Vitruvian Man, his famous drawing on human proportions. This illustrates the principle that in the shift between the spread-eagle pose and the straight pose, the apparent center of the figure seems to move, but in reality, the navel of the figure, which is the true center of gravity, remains motionless.Fact: date=June 2008

Lastly, Navels vary in height on the abdomen. An ideal proportion of navel height versus body height is said to be based on the Golden Section, also known as the "Divine Proportion" by philosophers and artists. This is a geometric proportion in which a line is divided so that the ratio of the length of the longer line segment to the length of the entire line is equal to the ratio of the length of the shorter line segment to the length of the longer line segment. This Golden Section ratio has a numerical value of approximately 1.618. In other words, an ideal navel height is about 62% of the body height and is said to exhibit special beauty as the legs and torso appear in sound proportion.