The Collaborative International Dictionary of English v.0.48:

Ataraxia \At`a*rax"i*a\, Ataraxy \At"a*rax`y\, n. [NL. ataraxia,
   Gr. 'ataraxi`a; 'a priv. + tarakto`s disturbed, tara`ssein to
   disturb.]
   Perfect peace of mind, or calmness.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "ataraxy":
   accidia, acedia, aloofness, apathy, ataraxia, benumbedness, blah,
   blahs, bovinity, calmness, carelessness, casualness, comatoseness,
   composure, contemplation, coolness, detachment, disinterest,
   dispassion, dispassionateness, disregard, disregardfulness,
   dullness, easy temper, easygoingness, even temper, good temper,
   heartlessness, hebetude, heedlessness, hopelessness, impassiveness,
   impassivity, imperturbability, imperturbableness, inappetence,
   inattention, incuriosity, indifference, indiscrimination,
   inexcitability, inexcitableness, inirritability, insouciance,
   lack of affect, lack of appetite, lackadaisicalness, languidness,
   lethargicalness, lethargy, listlessness, lucid stillness,
   marmoreal repose, mindlessness, negligence, nirvana, nonchalance,
   numbness, passiveness, passivity, patience, peace, peacefulness,
   phlegm, phlegmaticalness, phlegmaticness, placidity, placidness,
   plucklessness, pococurantism, quiescence, quiescency, quiet,
   quietism, quietness, quietude, recklessness, regardlessness,
   repose, resignation, resignedness, rest, restfulness, sangfroid,
   satori, serenity, silence, silken repose, sleep, sloth,
   sluggishness, slumber, smooth temper, sopor, soporifousness,
   spiritlessness, spunklessness, steadiness, stillness, stoicism,
   stolidity, stupefaction, stupor, supineness, torpidity, torpidness,
   torpor, tranquillity, unanxiousness, unconcern, unirritableness,
   unmindfulness, unnervousness, unpassionateness, unsolicitousness,
   wise passiveness, withdrawnness