n.1. a. The small projection near the center of the mammary gland containing the outlets of the milk ducts through which young mammals obtain milk from the adult female; a teat.
b. A corresponding projection of the male breast.
2. a. The rubber cap on a bottle from which a baby nurses.
b. A pacifier for an infant.
3. Any of various devices functioning like or resembling a nipple, especially:
a. A regulated opening for discharging a liquid, as in a small stopcock.
b. A pipe coupling threaded on both ends.
c. A short extension of pipe to which a nozzle can be attached.
d. A small projection through which grease in a grease gun can be forced into a bearing.
4. A natural or geographic projection resembling a nipple, as a mountain crest.