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. 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. The nipple is also used an anatomical landmark. It marks the T4 (fourth thoracic vertebra) dermatome and rests over the approximate level of the diaphram.
In the anatomy of mammals, a nipple, mammary papilla or teat is a small projection of skin containing the outlets for 15–20 lactiferous ducts arranged cylindrically around the tip. 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 innervation to the nipple is derived from the lateral cutaneous branches of fourth intercostal nerve. The physiological purpose of nipples is to deliver milk to the infant, produced in the female mammary glands during lactation. Marsupials and eutherian mammals typically have an even number of nipples arranged bilaterally, from as few as two to as many as 19.
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.