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]

WordNet (r) 3.0 (2006):

ataraxia
    n 1: peace of mind