ataraxia
noun
at·a·rax·ia ˌa-tə-ˈrak-sē-ə 
variants or less commonly ataraxy
ˈa-tə-ˌrak-sē
: calmness untroubled by mental or emotional disquiet
The highest goal of an Epicurean disciple was ataraxia—tranquility of mind.
Erich Segal
Erich SegalHermann Cohen contrasted the prophets' appreciation of the urgencies of history to the teachings of Stoicism enjoining ataraxia, the ideal of tranquillity to be attained by freeing oneself from emotional concerns and anxiety. Paul Mendes-Flohr
Paul Mendes-Flohr ataraxic
ˌa-tə-ˈrak-sik
adjective 
Word History
Etymology
borrowed from Middle French & Greek; Middle French ataraxie, borrowed from Greek ataraxía, from atáraktos "calm, unexcited" (from a- a- entry 2 + taraktós, verbal adjective of tarássein, (Attic) taráttein "to stir up, disturb, agitate mentally or emotionally") + -ia -y entry 2; tarássein going back to Indo-European *dhr̥h2gh-i̯é-, zero grade ablaut derivative of *dhreh2gh- "be stirred up, become turbulent," whence also Old Church Slavic razdražǫ, razdražiti "to provoke, make angry," Lithuanian dérgiu, dérgti "to make dirty, besmirch, defame"
Note: Greek tarássein has an alternate present thrássein/thráttein, aorist infinitive thrâxai, formed perhaps from an accentual variant *dhŕ̥h2gh-.
First Known Use
1881, in the meaning defined above
Medical Definition
ataraxia
noun
at·a·rax·ia ˌat-ə-ˈrak-sē-ə
variants or ataraxy
ˈat-ə-ˌrak-sē
plural ataraxias or ataraxies
: calmness untroubled by mental or emotional disquiet