In anatomy, a nipple, or mammary papilla, is a small projection of skin containing the outlets for 15-20 lactiferous ducts arranged cylindrically around the tip. The physiological purpose of nipples is to deliver milk during lactation by the female; in the male, nipples are vestigial. Mammalian infants have a rooting instinct for seeking the nipple, and a sucking instinct for extracting milk; in humans, this is called breastfeeding.

In human anatomy, the nipple is located near the center of the breast, surrounded by an area of sensitive, pigmented skin known as the areola. Mammals typically have an even number of nipples arranged bilaterally. Rarely, as in the platypus, the mammary glands empty onto the skin.

Embryologically, nipples develop along the 'milk lines' which in humans extend from the axilla (armpit) down to the pubis (groin) on both sides. Most people develop two nipples (one on each breast) but some have supernumerary nipples. Occasionally, these have lactiferous glands attached.

Small non-striated myocytes (muscle cells) arranged cylindrically within the nipple are responsible for the nipple becoming erect when they are stimulated (for example, by suckling).

Sometimes, babies (male or female) are born producing milk. This is called "witches' milk", is caused by maternal estrogens acting on the baby and is quite normal. Witches' milk disappears after several days.

Nipple is also the name given to things resembling the mammalian nipple, such as the tip of an artificial teat or the tip of a grease secreting mechanism in machinery.

In plumbing, a nipple is a short piece of threaded tubing used as a coupling.

For bicycles, a nipple is a nut placed at the end of each wheel spoke.

An Eastern Asian instrument known as a nipple gong is so named because of its similar shape to a nipple.