nip·ple 
(nĭpəl)
(nĭpəl)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.
[From obsolete neble, diminutive of NEB.]