The Collaborative International Dictionary of English v.0.48:

Fluid \Flu"id\, n.
   A fluid substance; a body whose particles move easily among
   themselves.
   [1913 Webster]

   Note: Fluid is a generic term, including liquids and gases as
         species. Water, air, and steam are fluids. By analogy,
         the term was sometimes applied to electricity and
         magnetism, as in phrases electric fluid, magnetic
         fluid, though not strictly appropriate; such usage has
         disappeared.
         [1913 Webster +PJC]

   Fluid dram, or Fluid drachm, a measure of capacity equal
      to one eighth of a fluid ounce.

   Fluid ounce.
   (a) In the United States, a measure of capacity, in
       apothecaries' or wine measure, equal to one sixteenth of
       a pint or 29.57 cubic centimeters. This, for water, is
       about 1.04158 ounces avoirdupois, or 455.6 grains.
   (b) In England, a measure of capacity equal to the twentieth
       part of an imperial pint. For water, this is the weight
       of the avoirdupois ounce, or 437.5 grains.

   Fluids of the body. (Physiol.) The circulating blood and
      lymph, the chyle, the gastric, pancreatic, and intestinal
      juices, the saliva, bile, urine, aqueous humor, and muscle
      serum are the more important fluids of the body. The
      tissues themselves contain a large amount of combined
      water, so much, that an entire human body dried in vacuo
      with a very moderate degree of heat gives about 66 per
      cent of water.

   Burning fluid, Elastic fluid, Electric fluid, Magnetic
   fluid, etc. See under Burning, Elastic, etc.
      [1913 Webster]

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]

WordNet (r) 3.0 (2006):

fluid ounce
    n 1: a British imperial unit of capacity or volume (liquid or
         dry) equal to 8 fluid drams or 28.416 cubic centimeters
         (1.734 cubic inches) [syn: fluidounce, fluid ounce]
    2: a United States unit of capacity or volume equal to 1.804
       cubic inches [syn: fluidounce, fluid ounce]