The Collaborative International Dictionary of English v.0.48:

Ounce \Ounce\ (ouns), n. [F. once, fr. L. uncia a twelfth, the
   twelfth part of a pound or of a foot: cf. Gr. 'o`gkos bulk,
   mass, atom. Cf. 2d Inch, Oke.]
   1. A unit of mass or weight, the sixteenth part of a pound
      avoirdupois, and containing 28.35 grams or 4371/2 grains.
      [1913 Webster +PJC]

   2. (Troy Weight) The twelfth part of a troy pound; one troy
      ounce weighs 31.103486 grams, 8 drams, or 480 grains.
      [1913 Webster +PJC]

   Note: The troy ounce contains twenty pennyweights, each of
         twenty-four grains, or, in all, 480 grains, and is the
         twelfth part of the troy pound. The troy ounce is also
         a weight in apothecaries' weight. [Troy ounce is
         sometimes written as one word, troyounce.]
         [1913 Webster]

   3. Fig.: A small portion; a bit. [Obs.]
      [1913 Webster]

            By ounces hung his locks that he had. --Chaucer.
      [1913 Webster]

   Fluid ounce. See under Fluid, n.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ounce \Ounce\, n. [F. once; cf. It. lonza, Sp. onza; prob. for
   lonce, taken as l'once, fr. L. lynx, Gr. ?, or an (assumed)
   fem. adj. lyncea, from lynx. Cf. Lynx.] (Zool.)
   A feline quadruped (Felis irbis syn. Felis uncia)
   resembling the leopard in size, and somewhat in color, but it
   has longer and thicker fur, which forms a short mane on the
   back. The ounce is pale yellowish gray, with irregular dark
   spots on the neck and limbs, and dark rings on the body. It
   inhabits the lofty mountain ranges of Asia. Called also
   once.
   [1913 Webster] Ounded

WordNet (r) 3.0 (2006):

ounce
    n 1: a unit of apothecary weight equal to 480 grains or one
         twelfth of a pound [syn: ounce, troy ounce,
         apothecaries' ounce]
    2: a unit of weight equal to one sixteenth of a pound or 16
       drams or 28.349 grams [syn: ounce, oz.]
    3: large feline of upland central Asia having long thick whitish
       fur [syn: snow leopard, ounce, Panthera uncia]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "ounce":
   ace, atom, bit, crumb, dab, dole, dot, dram, dribble, driblet,
   drop, dwarf, farthing, fleck, flyspeck, fragment, gobbet, grain,
   granule, groat, hair, handful, iota, jot, little, little bit,
   minim, minimum, minutiae, mite, modicum, molecule, mote, nutshell,
   particle, pebble, pinch, pittance, point, scruple, shred, smidgen,
   smitch, speck, spoonful, spot, thimbleful, tiny bit, tittle,
   trifling amount, trivia, whit



Bouvier's Law Dictionary, Revised 6th Ed (1856):

OUNCE. The name of a weight. An ounce avoirdupois weight is the sixteenth
part of a pound; an ounce troy weight is the twelfth part of a pound. Vide
Weights.