A state of freedom from disturbance of mind.
[From Greek ataraktos (not disturbed), from tarassein (to disturb).]
"Once ataraxia has been achieved, happiness cannot be augmented, either by more accomplishments or by a longer life."
David Voron; When Death Comes Knockin' Who Ya Gonna Call?; Skeptic (Altadena, California); Jan 2005.