The nipple is a raised region of tissue on the surface of the breast from which, in lactating females, milk from the mammary gland leaves the body through the lactiferous ducts to nurse an infant.

[1][2] The milk can flow through the nipple passively, or it can be ejected by smooth muscle contractions that occur along with the ductal system.

[7][8] In mammals, a nipple (also called mammary papilla or teat) is a small projection of skin containing the outlets for 15–20 lactiferous ducts arranged cylindrically around the tip.

[9] The skin of the nipple is rich in a supply of special nerves that are sensitive to certain stimuli: these are slowly-adapting and rapidly-adapting cutaneous mechanoreceptors.

It marks the T4 (fourth thoracic vertebra) dermatome and rests over the approximate level of the diaphragm.

The rest of the drainage leaves the nipple and breast through infroclavicular, pectoral, or parasternal nodes.

The physiological purpose of nipples is to deliver milk, produced in the female mammary glands during lactation, to an infant.

Oxytocin is a hormone that increases during pregnancy and acts on the breast to help produce the milk-ejection reflex.

Oxytocin release from the nipple stimulation of the infant causes the uterus to contract even after childbirth.

A poor latch results in insufficient nipple stimulation to create the let down reflex.

This poor attachment can cause sore and cracked nipples and a reluctance of the mother to continue to breastfeed.

A very thin, hollow needle and slight suction will be used to remove a small sample from under the nipple.

The nipple may swell in some men possibly due to increased levels of estrogen.

The average projection and size of human female nipples is slightly more than 3⁄8 inch (9.5 mm).

In these circumstances, the nipple itself can become infected with Candida that is present in the mouth of the breastfeeding infant.

[35] In some cases, if the mother has an infection with no nipple cracks or ulcerations, it is still safe to breastfeed the infant.

A nipple-sparing/subcutaneous mastectomy is a surgical procedure where breast tissue is removed, but the nipple and areola are preserved.

This procedure was historically done only prophylactically or with mastectomy for the benign disease over the fear of increased cancer development in retained areolar ductal tissue.

Recent series suggest that it may be an oncologically sound procedure for tumours not in the subareolar position.

[4][5][42] As female nipples are often perceived an intimate part, covering them might have originated under Victorian morality as with riding side saddle.

[42][43] The exposure of nipples is usually considered immodest and in some instances is viewed as lewd or indecent behavior.

[44] A case in Erie, Pennsylvania, concerning the exposure of breasts and nipple proceeded to the United States Supreme Court.

Other jurisdictions prohibit any female chest anatomy by banning anatomical structures that lie below the top of the areola or nipple.

West Virginia's regulation is very specific and is not likely to be misinterpreted, stating: "[The] display of 'any portion of the cleavage of the human female breast exhibited by a dress, blouse, skirt, leotard, bathing suit, or other wearing apparel [is permitted] provided the areola is not exposed, in whole or in part.

'"[43] The Instagram social media site has a "no nipples" policy with exceptions: material that is not allowed includes "some photos of female nipples, but photos of post-mastectomy scarring and women actively breastfeeding are allowed.

Instagram removed images of Rihanna and had her account cancelled in 2014 when she posted selfies with nipples.

[48][51][52] Filmmaker Lina Esco made a film entitled Free the Nipple, which is about "laws against female toplessness or restrictions on images of female, but not male, nipples", which Esco states is an example of sexism in society.