The Collaborative International Dictionary of English v.0.48:

Intensity \In*ten"si*ty\, n. [LL. intensitas: cf. F.
   intensit['e]. See Intense.]
   [1913 Webster]
   1. The state or quality of being intense; intenseness;
      extreme degree; as, intensity of heat, cold, mental
      application, passion, etc.
      [1913 Webster]

            If you would deepen the intensity of light, you must
            be content to bring into deeper blackness and more
            distinct and definite outline the shade that
            accompanies it. --F. W. Robertson.
      [1913 Webster]

   2. (Physics) The amount or degree of energy with which a
      force operates or a cause acts; effectiveness, as
      estimated by results produced.
      [1913 Webster]

   3. (Mech.) The magnitude of a distributed force, as pressure,
      stress, weight, etc., per unit of surface, or of volume,
      as the case may be; as, the measure of the intensity of a
      total stress of forty pounds which is distributed
      uniformly over a surface of four square inches area is ten
      pounds per square inch.
      [1913 Webster]

   4. (Photog.) The degree or depth of color or shade in a
      picture.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intensity
    n 1: the amount of energy transmitted (as by acoustic or
         electromagnetic radiation); "he adjusted the intensity of
         the sound"; "they measured the station's signal strength"
         [syn: intensity, strength, intensity level]
    2: high level or degree; the property of being intense [syn:
       intensity, intensiveness]
    3: the magnitude of sound (usually in a specified direction);
       "the kids played their music at full volume" [syn: volume,
       loudness, intensity] [ant: softness]
    4: chromatic purity: freedom from dilution with white and hence
       vivid in hue [syn: saturation, chroma, intensity,
       vividness]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "intensity":
   ASA scale, British candle, Hefner candle, Scheiner scale,
   Technicolor, abandon, acuteness, ampleness, amplitude, animality,
   ardency, ardor, atrocity, barbarity, bloodlust, bougie decimale,
   boundlessness, bright color, brightness, brilliance, brutality,
   bulk, candle, candle lumen, candle power, candle-foot, candle-hour,
   candle-meter, color, colorfulness, commitment, committedness,
   concentration, crescendo, decimal candle, dedication,
   demonic energy, destructiveness, devotedness, devotion, devoutness,
   dynamism, earnestness, energy, enormity, enormousness, ergal,
   expanse, exposure meter, extremity, faith, faithfulness,
   ferociousness, fervency, fervidness, fervor, fidelity, fierceness,
   fire, flux, focus, foot-candle, force, forcefulness,
   formidableness, fullness, furiousness, gaiety, gigantism,
   gorgeousness, grandeur, grandness, great scope, greatness,
   harshness, heartiness, heat, heatedness, hugeness, immensity,
   impassionedness, impetuosity, inclemency, infinity, inhumanity,
   intentness, international candle, kinetic energy, lamp-hour,
   largeness, light, light meter, light quantum, loudishness,
   loudness, loyalty, lumen, lumen meter, lumen-hour, lumeter,
   luminous flux, luminous intensity, luminous power, lux, magnitude,
   malignity, mercilessness, might, mightiness, mindlessness,
   muchness, murderousness, passion, passionateness, photon,
   pitilessness, plenitude, potency, potential energy, power,
   prodigiousness, pure color, quantum, resolution, richness, rigor,
   roughness, saturation, savagery, seriousness, severity, sharpness,
   sincerity, sonority, sonorousness, spirit, strength, strenuousness,
   stupendousness, surge, surge of sound, swell, swelling, terrorism,
   tremendousness, ungentleness, unit of flux, unit of light,
   vandalism, vastness, vehemence, venom, viciousness, vigor,
   violence, virulence, vitality, vividness, volume, warmth, zeal