(1):

(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.

(2):

(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.

(3):

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

(4):

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