The nipple is a raised region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant.^[1][2] The milk can flow through the nipple passively or it can be ejected by smooth muscle contractions that occur along the ductal system. The nipple is surrounded by the areola, which is often a darker color than the surrounding skin.^[3] A nipple is often called a teat when referring to non-humans. Nipple or teat can also be used to describe the flexible mouthpiece of a baby bottle. In humans, nipples of both males and females can be stimulated as part of sexual arousal. In many cultures, human female nipples are sexualized,^[4] or "...regarded as sex objects and evaluated in terms of their physical characteristics and sexiness."^[5]

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. Marsupials and eutherian mammals typically have an even number of nipples arranged bilaterally, from as few as two to as many as 19.^[6]

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. Mechanoreceptors are identified respectively by Type I slowly-adapting with multiple Merkel corpuscle end-organs and Type II slowly-adapting with single Ruffini corpuscle end-organs, as well as Type I rapidly-adapting with multiple Meissner corpuscle end-organs and Type II rapidly-adapting with single Pacinian corpuscle end-organs. The dominant nerve supply to the nipple comes from the lateral cutaneous branches of fourth intercostal nerve.^[7] The nipple is also used as an anatomical landmark. It marks the T4 (fourth thoracic vertebra) dermatome and rests over the approximate level of the diaphragm.^[8]

The arterial supply to the nipple and breast originates from the anterior intercostal branches of the internal thoracic (mammary) arteries; lateral thoracic artery; and thoracodorsal arteries.

The venous vessels parallel the arteries.^[2] The lymphatic ducts that drain the nipple are the same for the breast.^[2] The axillary nodes are the apical axillary nodes, the lateral group and the anterior group.^[9]

Since nipples change throughout the life span in men and women, the anatomy of the nipple can change and this change may be expected and considered normal.

Almost all mammals have nipples. Why males have nipples has been the subject of scientific research. Differences among the sexes (called sexual dimorphism) within a given species are considered by evolutionary biologists to be mostly the result of sexual selection, directly or indirectly. There is a general consensus that the male nipple exists because there is no particular advantage to males losing the trait (this called a spandrel).^[10][11][12]

The physiological purpose of nipples is to deliver milk to the infant, produced in the female mammary glands during lactation. During breastfeeding, nipple stimulation by an infant will simulate the release of oxytocin from the hypothalamus. 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.^[13][14] The strong uterine contractions that are caused by the stimulation of the mother's nipples help the uterus contract to clamp down the uterine arteries. These contractions are necessary to prevent post-partum hemorrhage.^[15]

When the baby suckles or stimulates the nipple, oxytocin levels rise and small muscles in the breast contract and move the milk through the milk ducts.

The result of nipple stimulation by the newborn helps to move breast milk out through the ducts and to the nipple.

This contraction of milk is called the “let-down reflex.”^[16] Latching on refers to the baby fastening onto the nipple to breastfeeding.

A good attachment is when the bottom of the areola (the area around the nipple) is in the baby's mouth and the nipple is drawn back inside his or her mouth.

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

The nipple is poorly stimulated when the baby latches on too close to the tip of the nipple.

This poor attachment can cause sore and cracked nipples and a reluctance of the mother to continue to breastfeed.^[17][18] After the birth of the infant, the milk supply increases based upon the continuous and increasing stimulation of the nipple by the infant. If the baby increases nursing time at the nipple, the mammary glands respond to this stimulation by increasing milk production.

Nipple pain can be a disincentive for breastfeeding.^[19] Sore nipples that progress to cracked nipples is of concern since many woman cease breastfeeding due to the pain. In some instances an ulcer will form on the nipple.^[20] One reason for the development of cracked and sore nipples is the incorrect latching-on of the infant to the nipple. If a nipple appears to be wedge-shaped, white and flattened, this may indicates that the attachment of the infant is not good and there is a potential of developing cracked nipples.^[21] Herpes infection of the nipple is painful.^[22] Nipple pain can also be caused by excessive friction of clothing against the nipple that causes a fissure.

Nipple discharge refers to any fluid that seeps out of the nipple of the breast.

Discharge from the nipple does not occur in lactating women.

And discharge in non-pregnant women or women who are not breastfeeding may not cause concern.

Men that have discharge from their nipples are not typical.

Discharge from the nipples of men or boys may indicate a problem.

Discharge from the nipples can appear without squeezing or may only be noticeable if the nipples are squeezed.

One nipple can have discharge while the other does not.

The discharge can be clear, green, bloody, brown or straw-colored.

The consistency can be thick, thin, sticky or watery.^[23][24]

Some cases of nipple discharge will clear on their own without treatment.

Nipple discharge is most often not cancer (benign), but rarely, it can be a sign of breast cancer.

It is important to find out what is causing it and to get treatment.

Here are some reasons for nipple discharge:^[23]

Sometimes, babies can have nipple discharge.

This is caused by hormones from the mother before birth.

It usually goes away in 2 weeks.

Cancers such as Paget disease (a rare type of cancer involving the skin of the nipple) can also cause nipple discharge.^[23]

Nipple discharge that is not normal is bloody, comes from only one nipple, or comes out on its own without squeezing or touching the nipple.

Nipple discharge is more likely to be normal if it comes out of both nipples or happens when the nipples are squeezed.

Squeezing the nipple to check for discharge can make it worse.

Leaving the nipple alone may make the discharge stop.^[23]

Any nipple discharge in a male usually is of more concern.

Most of the time a mammogram and an examination of the fluid is done.

Oftentimes a biopsy is performed.

A fine needle aspiration (FNA) biopsy can be fast and least painful.

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

Using a local anesthetic to numb the skin may not be necessary since a thin needle is used for the biopsy.

Receiving an injection to prevent pain from the biopsy may be more painful than the biopsy itself.^[25]

Some men develop a condition known as gynecomastia, in which the breast tissue under the nipple develops and grows. Discharge from the nipple can occur. The nipple may swell in some men possibly due to increased levels of estrogen.

Changes in appearance may be normal or related to disease.

The average projection and size of human female nipples is slightly more than 3⁄8 inch (9.5 mm).^[28]

Symptoms of breast cancer can often be seen first by changes of the nipple and areola, although not all women have the same symptoms, and some people do not have any signs or symptoms at all. A person may find out they have breast cancer after a routine mammogram. Warning signs can be:^[29][30]

Changes in the nipple are not necessarily symptoms or signs of breast cancer.

Other conditions of the nipple can mimic the signs and symptoms of breast cancer.^[29]

Some infections are transmitted through the nipple, especially if irritation or injury to the nipple has occurred.

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

The infant will transmit the infection to the mother.

Most of the time, this infection is localized to the area of the nipple.

In some cases the infection can progress to become a full-blown case of mastitis or breast infection.^[31] In some cases, if the mother has an infection with no nipple cracks or ulcerations, it is still safe to breastfeed the infant.

Herpes infection of the nipple can go unnoticed because the lesions are small but usually are quite painful.

Herpes in the newborn is a serious and sometimes fatal infection.^[22] Transmission of Hepatitis C and B to the infant can occur if the nipples are cracked.^[32]

Other infections can be transmitted through a break of the skin of the nipple and can infect 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 benign disease over fear of increased cancer development in retained areolar ductal tissue. Recent series suggest that it may be an oncologically sound procedure for tumors not in the subareolar position.^[35][36][37]

The culture tendency to hide the female nipple under clothing has existed in Western culture since the 1800s.^[38][5][4] As female nipples are often perceived an intimate part, covering them might originate as a Victorian taboo just as was riding side saddle. Exposing the entire breast and nipple is a form of protest for some and a crime for others.^[39][38] The exposure of nipples is usually considered immodest and in some instances is viewed as lewd or indecent behavior.^[40]

A case in Erie, Pennsylvania concerning the exposure of breasts and nipple proceeded to the Supreme Court in the US.^[41] The Erie ordinance was regulating the nipple in public as an act that is committed when a person "'... knowingly or intentionally,... appears in a state of nudity commits Public Indecency.'" Later in the statute, nudity is further described as an uncovered female nipple.

But nipple exposure of a man was not regulated.

A commentator expressed this opinion on the statute by noting: "Ponder the significance of that.

A man walks around bare-chested and the worst that happens is he won't get served in restaurants.

But a woman who goes topless is legally in the same boat as if she'd had sex in public.

That may seem crazy, but in the U.S. it's a permissible law.

— Cecil Adams"^[39]

The legality around the exposure of nipples are inconsistently regulated throughout the US.

Some states do not allow the visualization of any part of the breast.

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

Such is the case in West Virginia and Massachusetts.

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.'"^[39]

Instagram 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. Nudity in photos of paintings and sculptures is OK, too".^[42] Previously, Instagram had removed images of nursing mothers. Instagram removed images of Rihanna and had her account cancelled when she posted selfies with nipples. This was incentive for the Twitter campaign #FreeTheNipple.^[43] Another recent development is the Instagram page that invites users to post images of nipples from both sexes; @genderless_nipples, which displays close ups of both the nipples of men and women for the purpose of spotlighting what may be inconsistency.^[44] Some contributors have found 'a way around' this policy.^[45][46] Facebook has also been struggling to define its nipple policy.^[44][47][48]

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.^[49]

Nipples can be sensitive to touch, and nipple stimulation can incite sexual arousal.^[50] Few women report experiencing orgasm from nipple stimulation.^[51][52] Before Komisaruk et al.'s functional magnetic resonance (fMRI) research on nipple stimulation in 2011, reports of women achieving orgasm from nipple stimulation relied solely on anecdotal evidence.^[53] Komisaruk's study was the first to map the female genitals onto the sensory portion of the brain; it indicates that sensation from the nipples travels to the same part of the brain as sensations from the vagina, clitoris and cervix, and that these reported orgasms are genital orgasms caused by nipple stimulation, and may be directly linked to the genital sensory cortex ("the genital area of the brain").^[53][54][55]

Some companies and non-profit organisations have used the word nipple or images of nipples to draw attention to their product or cause.^[49][56]

The word "nipple" most likely originates as a diminutive of neb, an Old English word meaning "beak", "nose", or "face", and which is of Germanic origin.^[57] The words "teat" and "tit" share a Germanic ancestor. The second of the two, tit, was inherited directly from Proto-Germanic, while the first entered English via Old French.^[58][59]