Source : Webster's Revised Unabridged Dictionary (1913)

Tranquillity \Tran*quil"li*ty\, n. [F. tranquillit['e], L.
   tranquillitas.]
   The quality or state of being tranquil; calmness; composure.

Source : WordNet®

tranquillity
     n 1: an untroubled state; free from disturbances [syn: {quiet}]
     2: a state of peace and quiet [syn: {tranquility}, {quietness},
         {quietude}]
     3: a disposition free from stress or emotion [syn: {repose}, {quiet},
         {placidity}, {serenity}, {tranquility}]