The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   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.
          [1913 Webster]

   2. That which can be increased, diminished, or measured;
      especially (Math.), anything to which mathematical
      processes are applicable.
      [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster]

            The quantity of extensive and curious information
            which he had picked up during many months of
            desultory, but not unprofitable, study. --Macaulay.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

300 Moby Thesaurus words for "quantity":
   Alexandrine, a mass of, a world of, abundance, accent,
   accentuation, accommodation, account, acres, affluence, aggregate,
   amount, amphibrach, amphimacer, ample sufficiency, ampleness,
   amplitude, anacrusis, anapest, antispast, armful, army, arsis,
   avalanche, bacchius, bagful, bags, barometer, barrelful, barrels,
   basketful, batch, beat, bevy, binful, block, bonanza, bottleful,
   bountifulness, bountiousness, bowlful, box score, budget, bulk,
   bumper crop, bunch, burden, bushel, cadence, caesura, canon,
   capacity, capful, caseful, cast, catalexis, check, chloriamb,
   chloriambus, chunk, clod, cloud, clump, clutch, clutter, colon,
   content, copiousness, cordage, count, counterpoint, countlessness,
   covey, cretic, criterion, dactyl, dactylic hexameter, deal, degree,
   diaeresis, difference, dimeter, dipody, dochmiac, dose, elegiac,
   elegiac couplet, elegiac pentameter, emphasis, epitrite, extent,
   extravagance, exuberance, feminine caesura, fertility, flight,
   flock, flocks, flood, flow, foison, foot, full measure, full vowel,
   fullness, gauge, generosity, generousness, gob, gobs,
   graduated scale, great abundance, great plenty, group, gush, hail,
   handful, heap, heptameter, heptapody, heroic couplet, hexameter,
   hexapody, hive, host, hunk, iamb, iambic, iambic pentameter, ictus,
   ionic, jam, jillion, jingle, kettleful, landslide, lapful,
   large amount, lavishness, legion, liberality, liberalness, lilt,
   limit, load, loads, loaf, long vowel, lot, lots, lump, luxuriance,
   many, masculine caesura, mass, masses of, maximum, measure, mess,
   meter, metrical accent, metrical foot, metrical group,
   metrical unit, metron, million, mob, model, molossus, mora,
   more than enough, mountain, mouthful, movement, much, muchness,
   multitude, myriad, myriads, nest, norm, nugget, number, numbers,
   numerousness, ocean, oceans, oodles, opulence, opulency,
   outpouring, overflow, pack, paeon, parameter, parcel, part, pat,
   pattern, peck, pentameter, pentapody, period, plenitude,
   plenteousness, plentifulness, plenty, plurality, portion, poundage,
   prevalence, proceleusmatic, prodigality, product, productiveness,
   profuseness, profusion, pyrrhic, quantities, quantum, quite a few,
   ration, reading, readout, reckoning, reduced vowel, repleteness,
   repletion, rhythm, rich harvest, rich vein, richness, riot,
   riotousness, room, rout, ruck, rule, scads, scale, score, scores,
   sea, shoal, short vowel, shower, small amount, space, spate,
   spondee, sprung rhythm, standard, stowage, stream, stress,
   substantiality, substantialness, sum, summation, superabundance,
   superfluity, swarm, swing, syzygy, tale, tally, teemingness, test,
   tetrameter, tetrapody, tetraseme, the bottom line, the story,
   the whole story, thesis, thousand, throng, tidy sum, tonnage, tons,
   total, touchstone, tribrach, trillion, trimeter, tripody, triseme,
   trochee, type, value, volume, wad, wealth, weight, whole, world,
   worlds, worlds of, x number, yardstick



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

QUANTITY, pleading. That which is susceptible of measure.
     2. It is a general rule that, when the declaration alleges an injury to
goods and chattels, or any contract relating to them, their quantity should
be stated. Gould on Pl. c. 4, Sec. 35. And in actions for the recovery of
real estate, the quantity of the land should be specified. Bract. 431, a; 11
Co. 25 b, 55 a; Doct. Pl. 85, 86; 1 East, R. 441; 8 East, R. 357; 13 East,
R. 102; Steph. Pl. 314, 315.