The Collaborative International Dictionary of English v.0.48:

Passivity \Pas*siv"i*ty\, n. [Cf. F. passivit['e].]
   1. Passiveness; -- opposed to activity. --Jer. Taylor.
      [1913 Webster]

   2. (Physics) The tendency of a body to remain in a given
      state, either of motion or rest, till disturbed by another
      body; inertia. --Cheyne.
      [1913 Webster]

   3. (Chem.) The quality or condition of any substance which
      has no inclination to chemical activity; inactivity.
      [1913 Webster]

WordNet (r) 3.0 (2006):

passivity
    n 1: the trait of remaining inactive; a lack of initiative [syn:
         passivity, passiveness]
    2: submission to others or to outside influences [syn:
       passivity, passiveness]

Moby Thesaurus II by Grady Ward, 1.0:

169 Moby Thesaurus words for "passivity":
   Quakerism, a wise passiveness, abeyance, acceptance, acedia,
   acquiescence, aloofness, amenability, apathy, assent, ataraxia,
   ataraxy, benumbedness, blah, blahs, blank mind, blankmindedness,
   blankness, boredom, calm of mind, catalepsy, catatonia,
   comatoseness, complaisance, compliance, consent, contemplation,
   contemplative life, deadliness, deathliness, deference, detachment,
   disinterest, dispassion, do-nothing policy, do-nothingism,
   do-nothingness, dormancy, drowsiness, dullness, emptiness of mind,
   empty-headedness, enervation, ennui, entropy, fallow mind, fatigue,
   fatuity, foolishness, heartlessness, heaviness, hebetude, homage,
   hopelessness, humbleness, humility, idleness, immobility, inaction,
   inactivity, inanimation, inanity, inappetence, indifference,
   indolence, inertia, inertness, insouciance, jadedness, kneeling,
   lack of appetite, lackadaisicalness, laissez-aller, laissez-faire,
   laissez-faireism, languidness, languishment, languor,
   languorousness, lassitude, latency, lenitude, lentor,
   lethargicalness, lethargy, lifelessness, listlessness,
   lotus-eating, meditation, meekness, mental blankness, neutralism,
   neutrality, neutralness, nirvana, nonchalance, noninvolvement,
   nonopposal, nonopposition, nonparticipation, nonresistance,
   nonviolence, nonviolent resistance, numbness, obedience, obeisance,
   oblivion, oscitancy, pacifism, paralysis, passive resistance,
   passive self-annihilation, passiveness, passivism, phlegm,
   phlegmaticalness, phlegmaticness, plucklessness, policy,
   procrastination, quiescence, quietism, resignation, resignedness,
   satedness, sleepiness, sloth, slothfulness, slowness, sluggishness,
   somnolence, sopor, soporifousness, spiritlessness, spunklessness,
   stagnancy, stagnation, standpattism, stasis, stupefaction, stupor,
   subjection, submission, submissiveness, submittal, supineness,
   suspense, tabula rasa, thoughtfreeness, thoughtlessness, torpidity,
   torpidness, torpitude, torpor, tranquillity, uncomplainingness,
   unconcern, unintelligence, vacancy, vacuity, vegetation,
   vis inertiae, vita contemplativa, waiting game,
   watching and waiting, weariness, withdrawnness, world-weariness,
   yielding