nipple

n. tip of mammary gland of mammals; place out of which milk flows from the female breast; any nipple-shaped device (e.g. pacifier, bottle top, etc.); threaded projection for connecting pipes

In its most general form, a nipple is an appurtenance from which a fluid emanates, in this instance breast milk, to nurture a mother's young.
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Nipple /nip·ple/ 1. mammary papilla; the pigmented projection on the anterior surface of the breast, surrounded by the areola; in women it gives [more]Nipple - Community and Resources

Noun
1. the small projection of a mammary gland
(synonym) mammilla, mamilla, pap, teat, tit
(hypernym) reproductive organ, sex organ
(part-holonym) mammary gland, mamma

(n.)
The protuberance through which milk is drawn from the breast or mamma; the mammilla; a teat; a pap.
  

(n.)
The orifice at which any animal liquid, as the oil from an oil bag, is discharged.
  

(n.)
Any small projection or article in which there is an orifice for discharging a fluid, or for other purposes; as, the nipple of a nursing bottle; the nipple of a percussion lock, or that part on which the cap is put and through which the fire passes to the charge.
  

(n.)
A pipe fitting, consisting of a short piece of pipe, usually provided with a screw thread at each end, for connecting two other fittings.