In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts of a mammal by which breast milk is delivered to a mother's young.
Mammals typically have an even number of nipples arranged around bilaterally. They develop in the embryo, along the 'milk lines'. Most mammals develop multiple nipples along each milk line, with the total number approximating the maximum litter size, and half the total number (i.e. the number on one side) approximating the average litter size for that species. In the primitive mammals (monotremes such as the platypus), the mammary glands empty onto the skin without a nipple. In cetaceans such as whales, the infant cannot form a suction-seal to nurse, due to its mouth structure. Therefore the whale's nipple is unlike that of any other mammal. Rather than requiring a sucking action, the discharge of milk is powered by maternal muscles. The calf takes the extended nipple into its mouth, and the mother ejects or expels her milk into the mouth of the calf.
Most humans have two nipples after birth, located near the center of each breast and surrounded by an area of sensitive, pigmented skin known as the areola. Human fetuses develop several more nipples along the milk lines, which extend from the axilla (armpit), along the abdominal muscles, down to the pubis (groin) on both sides. Those nipples usually disappear before birth, but sometimes remain, resulting in supernumerary nipples which occasionally have lactiferous glands attached, but usually do not.
The pigments of the nipple and areola are brown eumelanin (a brown pigment) and to a greater extent pheomelanin (a red pigment). Exposure to cold temperatures and breastfeeding are primary causes for the nipples to become erect; nipple erections are a product of the pilomotor reflex which causes goose bumps. Sexual arousal may also cause the nipples to become erect. The nipple and areola of males and females can be erotic receptors. They are not, however sex organs because, since mammals of both sexes all have nipples, nipples don't possess the defining quality of a sex organ of differentiating the sex of any animal.
The average projection and size of human female nipples is slightly more than 3/8 inches (10mm).. Pregnancy and nursing tend to increase nipple size, sometimes permanently. Pregnancy also increases the pigmentation. The erection of the nipple is partially due to the cylindrically arranged muscle cells found within it. In many women, there are small bulges on the areola, which are called 'Montgomery bodies'.
Sometimes, babies (male or female) are born producing milk. This is called 'witch's milk'; it is caused by maternal estrogens acting on the baby and is quite common. Witch's milk disappears after several days.