Source : Webster's Revised Unabridged Dictionary (1913)

Quantity \Quan"ti*ty\, v. t. [L. quantus now much + -fy.]
   To modify or qualify with respect to quantity; to fix or
   express the quantity of; to rate.

Quantity \Quan"ti*ty\, n.; pl. {Quantities}. [F. quantite, L.
   quantitas, fr. quantus bow great, how much, akin to quam bow,
   E. how, who. See {Who}.]
   1. The attribute of being so much, and not more or less; the
      property of being measurable, or capable of increase and
      decrease, multiplication and division; greatness; and more
      concretely, that which answers the question ``How much?'';
      measure in regard to bulk or amount; determinate or
      comparative dimensions; measure; amount; bulk; extent;
      size. Hence, in specific uses:
      (a) (Logic) The extent or extension of a general
          conception, that is, the number of species or
          individuals to which it may be applied; also, its
          content or comprehension, that is, the number of its
          constituent qualities, attributes, or relations.
      (b) (Gram.) The measure of a syllable; that which
          determines the time in which it is pronounced; as, the
          long or short quantity of a vowel or syllable.
      (c) (Mus.) The relative duration of a tone.

   2. That which can be increased, diminished, or measured;
      especially (Math.), anything to which mathematical
      processes are applicable.

   Note: Quantity is discrete when it is applied to separate
         objects, as in number; continuous, when the parts are
         connected, either in succession, as in time, motion,
         etc., or in extension, as by the dimensions of space,
         viz., length, breadth, and thickness.

   3. A determinate or estimated amount; a sum or bulk; a
      certain portion or part; sometimes, a considerable amount;
      a large portion, bulk, or sum; as, a medicine taken in
      quantities, that is, in large quantities.

            The quantity of extensive and curious information
            which he had picked up during many months of
            desultory, but not unprofitable, study. --Macaulay.

   {Quantity of estate} (Law), its time of continuance, or
      degree of interest, as in fee, for life, or for years.
      --Wharton (Law Dict. )

   {Quantity of matter}, in a body, its mass, as determined by
      its weight, or by its momentum under a given velocity.

   {Quantity of motion} (Mech.), in a body, the relative amount
      of its motion, as measured by its momentum, varying as the
      product of mass and velocity.

   {Known quantities} (Math.), quantities whose values are
      given.

   {Unknown quantities} (Math.), quantities whose values are
      sought.

Source : WordNet®

quantity
     n 1: how much there is of something that you can quantify [syn: {measure},
           {amount}]
     2: an adequate or large amount; "he had a quantity of
        ammunition"
     3: something that has a magnitude and can be represented in
        mathematical expressions by a constant or a variable