The Collaborative International Dictionary of English v.0.48:

Amplitude \Am"pli*tude\, n. [L. amplitudo, fr. amplus: cf. F.
   amplitude. See Ample.]
   1. State of being ample; extent of surface or space;
      largeness of dimensions; size.
      [1913 Webster]

            The cathedral of Lincoln . . . is a magnificent
            structure, proportionable to the amplitude of the
            diocese.                              --Fuller.
      [1913 Webster]

   2. Largeness, in a figurative sense; breadth; abundance;
      fullness.
      (a) Of extent of capacity or intellectual powers.
          "Amplitude of mind." --Milton. "Amplitude of
          comprehension." --Macaulay.
      (b) Of extent of means or resources. "Amplitude of
          reward." --Bacon.
          [1913 Webster]

   3. (Astron.)
      (a) The arc of the horizon between the true east or west
          point and the center of the sun, or a star, at its
          rising or setting. At the rising, the amplitude is
          eastern or ortive: at the setting, it is western,
          occiduous, or occasive. It is also northern or
          southern, when north or south of the equator.
      (b) The arc of the horizon between the true east or west
          point and the foot of the vertical circle passing
          through any star or object.
          [1913 Webster]

   4. (Gun.) The horizontal line which measures the distance to
      which a projectile is thrown; the range.
      [1913 Webster]

   5. (Physics) The extent of a movement measured from the
      starting point or position of equilibrium; -- applied
      especially to vibratory movements.
      [1913 Webster]

   6. (math.) An angle upon which the value of some function
      depends; -- a term used more especially in connection with
      elliptic functions.
      [1913 Webster]

   Magnetic amplitude, the angular distance of a heavenly
      body, when on the horizon, from the magnetic east or west
      point as indicated by the compass. The difference between
      the magnetic and the true or astronomical amplitude (see 3
      above) is the "variation of the compass."
      [1913 Webster]

WordNet (r) 3.0 (2006):

amplitude
    n 1: (physics) the maximum displacement of a periodic wave
    2: the property of copious abundance [syn: amplitude,
       bountifulness, bounty]
    3: greatness of magnitude

Moby Thesaurus II by Grady Ward, 1.0:

244 Moby Thesaurus words for "amplitude":
   abundance, affluence, amount, ample sufficiency, ampleness,
   antinode, area, auditory effect, auditory phenomenon, avalanche,
   beam, bigness, body, bonanza, boundlessness, bountifulness,
   bountiousness, breadth, broadness, bulk, bumper crop, caliber,
   camber, capaciousness, capacity, cloud of words, commodiousness,
   comprehensiveness, congestion, copiousness, coverage, crest,
   de Broglie wave, depth, diameter, diffraction, diffuseness,
   diffusion, diffusiveness, dimension, dimensions, distance,
   distance across, effusion, effusiveness, electromagnetic radiation,
   electromagnetic wave, enormity, enormousness, expanse, expansion,
   expansiveness, extension, extensiveness, extent, extravagance,
   exuberance, fecundity, fertility, flight path, flood, flood tide,
   flow, fluency, foison, force, formidableness, formlessness,
   frequency, frequency band, frequency spectrum, full, full measure,
   fullness, gauge, generosity, generousness, gigantism, girth,
   grandeur, grandness, great abundance, great plenty, great scope,
   greatness, guided wave, gush, gushing, height, high tide,
   high water, hugeness, immensity, impletion, in phase, infinity,
   intensity, interference, landslide, largeness, latitude,
   lavishness, length, liberality, liberalness, light, logorrhea,
   longitudinal wave, lots, loudness, luxuriance, macrology,
   magnitude, mass, matter, maximum, measure, measurement,
   mechanical wave, might, mightiness, more than enough, much,
   muchness, myriad, myriads, node, noise, numbers, numerousness,
   opulence, opulency, out of phase, outpour, outpouring, overflow,
   overfullness, palilogy, period, periodic wave, phone, plenitude,
   plenteousness, plentifulness, plenty, pleonasm, plethora, power,
   prevalence, prodigality, prodigiousness, productiveness,
   productivity, profuseness, profusion, prolificacy, prolificity,
   proportion, proportions, quantities, quantity, quantum, radio wave,
   radius, rampancy, range, rankness, ray, reach, redundancy,
   reinforcement, reiteration, reiterativeness, repetition for effect,
   repetitiveness, repleteness, repletion, resonance,
   resonance frequency, rich harvest, rich vein, richness, riot,
   riotousness, roominess, satiety, saturation, saturation point,
   scads, scale, scope, seismic wave, shock wave, shower, size,
   skin effect, skin friction, slip, sonance, sound,
   sound intensity level, sound propagation, sound wave, space,
   spaciousness, span, spate, speech sound, spread, spring tide,
   stagger, stream, strength, stretch, stupendousness, substance,
   substantiality, substantialness, sum, superabundance, superfluity,
   superflux, surface wave, surfeit, talkativeness, tautology,
   teemingness, tidal wave, tirade, transverse wave, tremendousness,
   trough, ultrasound, vastness, volume, wave, wave equation,
   wave motion, wave number, wavelength, wealth, whole, wideness,
   width