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]