In mammals, a nipple is the small projection near the center of the mammary gland containing the outlets of the milk ducts through which the young obtain milk from the adult female. Nipples are found in human females and males. They are more pronounced in females, but can be aroused in both sexes. Nipples may be enlarged through training. Enlarged nipples can be pierced for jewelry.
In BDSM, a top may pinch the bottom's nipples (often, using nipple clamps or clothespins). A top may also spank or flog the bottom's nipples. Common implements used on women are hand, flogger, and riding crop. During sexual intercourse, the woman can ride the man, presenting her nipples to him for hand spanking. A man may be spanked on the nipples using many implements, but particularly a leather strap or riding crop. In both sexes, nipple spanking may be combined with spanking the torso and thighs.
The bottom may seek to cover the sensitive nipples with the hands. Thus, restraining the hands overhead is a common position. A blindfold prevents the bottom from seeing the strokes land. Trained nipples don't usually return to their former state, but remain more pronounced.
Nipples can also be erected/aroused using suction bulbs.