Source : Webster's Revised Unabridged Dictionary (1913)

Impassivity \Im`pas*siv"i*ty\, n.
   The quality of being insusceptible of feeling, pain, or
   suffering; impassiveness.

Source : WordNet®

impassivity
     n : apathy demonstrated by an absence of emotional reactions
         [syn: {emotionlessness}, {impassiveness}, {phlegm}, {indifference},
          {stolidity}, {unemotionality}]