The Collaborative International Dictionary of English v.0.48:

Dispassion \Dis*pas"sion\, n.
   Freedom from passion; an undisturbed state; apathy. --Sir W.
   Temple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dispassion
    n 1: objectivity and detachment; "her manner assumed a
         dispassion and dryness very unlike her usual tone" [syn:
         dispassion, dispassionateness, dryness]

Moby Thesaurus II by Grady Ward, 1.0:

176 Moby Thesaurus words for "dispassion":
   Olympian detachment, abnegation, abstinence, accidia, acedia,
   aloofness, anesthesia, apathy, ataraxia, ataraxy, autism,
   benumbedness, blah, blahs, bovinity, calm, calmness, carelessness,
   casualness, catatonia, chill, chilliness, cold blood, cold heart,
   coldheartedness, coldness, comatoseness, conservatism, constraint,
   continence, control, cool, coolness, deadpan, detachment,
   disinterest, disinterestedness, dispassionateness, disregard,
   disregardfulness, dullness, easy temper, easygoingness,
   emotional deadness, emotionlessness, even temper, evenness,
   frigidity, frostiness, gentleness, golden mean, good temper,
   happy medium, heartlessness, hebetude, heedlessness, hopelessness,
   iciness, immovability, impartiality, impassibility, impassiveness,
   impassivity, imperturbability, imperturbableness, inappetence,
   inattention, incuriosity, indifference, indiscrimination,
   inexcitability, inexcitableness, inirritability, insouciance,
   judiciousness, juste-milieu, lack of affect, lack of appetite,
   lack of feeling, lack of touch, lackadaisicalness, languidness,
   lenity, lethargicalness, lethargy, listlessness, loftiness,
   meden agan, middle way, mildness, mindlessness, moderateness,
   moderation, moderationism, negligence, neutrality, nonchalance,
   nonviolence, nothing in excess, numbness, objectivity, obtuseness,
   pacifism, passionlessness, passiveness, passivity, patience,
   phlegm, phlegmaticalness, phlegmaticness, plucklessness,
   pococurantism, poker face, prudence, recklessness, regardlessness,
   repose, resignation, resignedness, restraint, self-abnegation,
   self-absorption, self-control, self-denial, self-restraint,
   selflessness, serenity, sloth, sluggishness, smooth temper,
   sobriety, sopor, soporifousness, soullessness, spiritlessness,
   spunklessness, stability, steadiness, stoicism, stolidity,
   straight face, stupefaction, stupor, supineness, temperance,
   temperateness, torpidity, torpidness, torpor, tranquillity,
   unanxiousness, unbias, unbiasedness, unconcern, unemotionalism,
   unexcessiveness, unexcitability, unextravagance, unextremeness,
   unfeeling, unfeelingness, unimpressibility, unimpressionableness,
   unirritableness, unmindfulness, unnervousness, unpassionateness,
   unresponsiveness, unselfishness, unsolicitousness,
   unsusceptibility, unsympatheticness, untouchability, via media,
   withdrawal, withdrawnness