The Collaborative International Dictionary of English v.0.48:

Intense \In*tense"\, a. [L. intensus stretched, tight, p. p. of
   intendere to stretch: cf. F. intense. See Intend, and cf.
   Intent, and cf. Intent, a.]
   [1913 Webster]
   1. Strained; tightly drawn; kept on the stretch; strict; very
      close or earnest; as, intense study or application;
      intense thought.
      [1913 Webster]

   2. Extreme in degree; excessive; immoderate; as:
      (a) Ardent; fervent; as, intense heat.
      (b) Keen; biting; as, intense cold.
      (c) Vehement; earnest; exceedingly strong; as, intense
          passion or hate.
      (d) Very severe; violent; as, intense pain or anguish.
      (e) Deep; strong; brilliant; as, intense color or light.
          [1913 Webster]

                In this intense seclusion of the forest.
                                                  --Hawthorne.
          [1913 Webster]

WordNet (r) 3.0 (2006):

intense
    adj 1: possessing or displaying a distinctive feature to a
           heightened degree; "intense heat"; "intense anxiety";
           "intense desire"; "intense emotion"; "the skunk's intense
           acrid odor"; "intense pain"; "enemy fire was intense"
           [ant: mild]
    2: extremely sharp or intense; "acute pain"; "felt acute
       annoyance"; "intense itching and burning" [syn: acute,
       intense]
    3: (of color) having the highest saturation; "vivid green";
       "intense blue" [syn: intense, vivid]

Moby Thesaurus II by Grady Ward, 1.0:

212 Moby Thesaurus words for "intense":
   abandoned, ablaze, accentuated, active, acute, advertent, afire,
   aggravated, aggressive, agog, alert, all ears, all eyes, animated,
   ardent, assiduous, attentive, aware, boiling over, breathless,
   bright, bright-hued, brilliant, brisk, burning, careful, colorful,
   colory, committed, comprehensive, concentrated, conscious,
   consequential, considerable, consuming, cordial, cutting,
   deafening, dedicated, deep, deep-colored, delirious, desperate,
   devoted, devout, diligent, drastic, drunk, dynamic, eager, earnest,
   emotional, emphasized, energetic, enhanced, enterprising,
   enthusiastic, excessive, excited, excruciating, exhaustive,
   exorbitant, exotic, exquisite, extravagant, extreme, exuberant,
   faithful, fanatical, febrile, fervent, fervid, fevered, feverish,
   fierce, fiery, finical, finicking, finicky, flaming, flushed,
   forceful, forcible, frantic, frenzied, full, full of pep, furious,
   gay, gay-colored, glowing, go-go, gorgeous, grand, grave, great,
   hard, harsh, heartfelt, hearty, heated, heavy, heedful, heightened,
   high-colored, high-spirited, hot, hot-blooded, hotheaded,
   hysterical, immoderate, impassioned, impetuous, impulsive,
   in earnest, incisive, intemperate, intensified, intent, intent on,
   intentive, intoxicated, irresistible, keen, kinetic, lively,
   living, loud, loyal, lusty, main, maximum, meticulous, mettlesome,
   mighty, mindful, nervous, nice, niggling, observant, observing,
   on fire, on the ball, on the job, open-eared, open-eyed,
   openmouthed, outrageous, passionate, penetrating, peppy, perfervid,
   piercing, plenary, poignant, powerful, profound, pungent, red-hot,
   reeking, regardful, resolute, rich, rich-colored, rigorous, robust,
   rough, serious, severe, sharp, sincere, smacking, snappy, spanking,
   spirited, splitting, steaming, steamy, strenuous, stressed, strong,
   take-charge, take-over, temperamental, tense, terrible, testy,
   thundering, total, touchy, tough, trenchant, unconscionable,
   unrestrained, uptight, vehement, venomous, vibrant, vicious,
   vigorous, violent, virulent, vivacious, vivid, volatile, warm,
   watchful, white-hot, zealous, zestful, zesty, zippy