WordNet (r) 3.0 (2006):

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