The Collaborative International Dictionary of English v.0.48:

Breathless \Breath"less\ (br[e^]th"l[e^]s), a.
   1. Spent with labor or violent action; out of breath.
      [1913 Webster]

   2. Not breathing; holding the breath, on account of fear,
      expectation, or intense interest; attended with a holding
      of the breath; as, breathless attention.
      [1913 Webster]

            But breathless, as we grow when feeling most.
                                                  --Byron.
      [1913 Webster]

   3. Dead; as, a breathless body.
      [1913 Webster]

WordNet (r) 3.0 (2006):

breathless
    adj 1: not breathing or able to breathe except with difficulty;
           "breathless at thought of what I had done"; "breathless
           from running"; "followed the match with breathless
           interest" [syn: breathless, dyspneic, dyspnoeic,
           dyspneal, dyspnoeal] [ant: breathing, eupneic,
           eupnoeic]
    2: tending to cause suspension of regular breathing; "a
       breathless flight"; "breathtaking adventure" [syn:
       breathless, breathtaking]
    3: appearing dead; not breathing or having no perceptible pulse;
       "an inanimate body"; "pulseless and dead" [syn: breathless,
       inanimate, pulseless]

Moby Thesaurus II by Grady Ward, 1.0:

224 Moby Thesaurus words for "breathless":
   ablaze, abrupt, afire, agape, aghast, agog, airless, alacritous,
   all agog, amazed, anaudic, animated, antsy, antsy-pantsy, anxious,
   aphasic, aphonic, ardent, asleep, asleep in Jesus, astonished,
   astounded, at gaze, at rest, athirst, avid, awed, awestruck,
   beguiled, bereft of life, bewildered, bewitched, boiling over,
   breakneck, breezeless, burning, bursting to, called home,
   captivated, carrion, chafing, close, confounded, cordial, croaked,
   dead, dead and gone, death-struck, deceased, defunct, delirious,
   demised, departed, departed this life, desirous, destitute of life,
   done for, drunk, dumb, dumbfounded, dumbstricken, dumbstruck,
   eager, edgy, enchanted, enraptured, enravished, enthralled,
   enthusiastic, entranced, exanimate, excited, exhausted, exuberant,
   fallen, fascinated, febrile, fervent, fervid, fevered, feverish,
   fiery, finished, flabbergasted, flaming, flushed, food for worms,
   forward, fretful, fretting, full of life, gaping, gasping, gauping,
   gazing, glowing, gone, gone to glory, gone west, hasty, headlong,
   hearty, heated, hopped-up, hot, hypnotized, ill-ventilated,
   impassioned, impatient, impetuous, impulsive, in a lather,
   in a stew, in a sweat, in awe, in awe of, in suspense, inanimate,
   inarticulate, intense, intoxicated, keen, late, late lamented,
   launched into eternity, lifeless, lively, lost in wonder, martyred,
   marveling, mesmerized, mum, mute, no more, on edge, on fire,
   open-eyed, openmouthed, oppressive, out of breath, overwhelmed,
   panting, passed on, passionate, popeyed, precipitant, precipitate,
   precipitous, prompt, puffed, puffing, pushing up daisies, puzzled,
   quick, rapt in wonder, raring to, rash, ready, ready and willing,
   red-hot, released, reposing, resting easy, restive, restless,
   sainted, short-breathed, short-winded, sleeping,
   smitten with death, speechless, spellbound, spent, spirited,
   squirming, squirmy, staggered, staring, steaming, steamy, stifling,
   still, stillborn, stricken dumb, stuffy, stupefied, sudden,
   suffocating, sultry, surprised, taken away, taken off, thirsty,
   thunderstruck, tired out, tongue-tied, tongueless, under a charm,
   uneasy, unpatient, unquiet, unrestrained, unvented, unventilated,
   vigorous, vital, vivacious, vivid, voiceless, warm, wheezing,
   wide-eyed, winded, windless, with the Lord, with the saints,
   without life, without vital functions, wonder-struck, wondering,
   wordless, worn out, zealous, zestful