nipple

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun A corresponding projection of the male breast.
• noun The rubber cap on a bottle from which a baby nurses.
• noun A pacifier for an infant.
• noun Any of various devices functioning like or resembling a nipple, especially.
• noun A regulated opening for discharging a liquid, as in a small stopcock.
• noun A pipe coupling threaded on both ends.
• noun A short extension of pipe to which a nozzle can be attached.
• noun A small projection through which grease in a grease gun can be forced into a bearing.
• noun A natural or geographic projection resembling a nipple, as a mountain crest.

from The Century Dictionary.

• To furnish with a nipple or nipples; cover with nipple-like protuberances.
• noun A protuberance of the breast where, in the female, the galactophorous ducts discharge; a pap; a teat.
• noun The papilla by which any animal secretion is discharged.
• noun Anything that projects like a nipple, as the projecting piece in a gun or a cartridge upon which the percussion-cap is placed to be struck by the hammer, the mouthpiece of a nursing-bottle, a nipple-shield, etc.
• noun A reducing-coupling for hose or for joining a hose to a pipe.
• noun A hollow piece projecting from and forming a passage connecting with the interior of a metal pipe, used for the attachment of a faucet or cock.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) The protuberance through which milk is drawn from the breast or mamma; the mammilla; a teat; a pap.
• noun rare The orifice at which any animal liquid, as the oil from an oil bag, is discharged.
• noun Any small projection or article in which there is an orifice for discharging a fluid, or for other purposes
• noun (Mech.) A pipe fitting, consisting of a short piece of pipe, usually provided with a screw thread at each end, for connecting two other fittings.
• noun a short pipe, usually of brass, one end of which is tapered and adapted for attachment to the end of a lead pipe by soldering.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The projection of a mammary gland from which, on female mammals, milk is secreted.
• noun An artificial nipple (definition 1) used for bottle-feeding infants.
• noun A mechanical device through which liquids can be passed in a regulated manner.
• noun A short tube threaded at both ends, used as a connector.
• noun Any small physical protrusion, such as the lumps on the F and J keys on American keyboards.
• noun Any small physical protrusion on an automotive, a machine part or any other part that fits into a groove on another part.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a flexible cap on a baby's feeding bottle or pacifier
• noun the small projection of a mammary gland