The Collaborative International Dictionary of English v.0.48:

Nipple \Nip"ple\ (n[i^]p"p'l), n. [Formerly neble, a dim. of
   neb. See Neb, Nib.]
   1. (Anat.) The protuberance through which milk is drawn from
      the breast or mamma; the mammilla; a teat; a pap.
      [1913 Webster]

   2. The orifice at which any animal liquid, as the oil from an
      oil bag, is discharged. [R.] --Derham.
      [1913 Webster]

   3. 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.
      [1913 Webster]

   4. (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.
      [1913 Webster]

   Solder nipple, 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nipple
    n 1: the small projection of a mammary gland [syn: nipple,
         mammilla, mamilla, pap, teat, tit]
    2: a flexible cap on a baby's feeding bottle or pacifier

The Free On-line Dictionary of Computing (19 January 2023):

nipple

   Trackpoint