The Collaborative International Dictionary of English v.0.48:

Tranquillity \Tran*quil"li*ty\, n. [F. tranquillit['e], L.
   tranquillitas.]
   The quality or state of being tranquil; calmness; composure.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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