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]

U.S. Gazetteer (1990):

Tranquillity, CA
  Zip code(s): 93668

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "tranquillity":
   Buddha-like composure, Oriental calm, abnegation, abstinence,
   arrangement, array, ataraxia, ataraxy, awful silence, blank mind,
   blankmindedness, blankness, calm, calm disposition, calm of mind,
   calmness, comfort, composure, concord, conservatism, constraint,
   contemplation, continence, control, cool, coolheadedness, coolness,
   countenance, dead, dead of night, deathlike silence, deployment,
   dispassion, disposal, disposition, ease, easy mind,
   emptiness of mind, empty-headedness, evenness, fallow mind,
   fatuity, foolishness, formation, gentleness, golden mean,
   golden silence, happy medium, harmony, hush, hush of night,
   icy calm, impartiality, imperturbability, imperturbation,
   inactivity, inanity, inaudibility, indisturbance, judiciousness,
   juste-milieu, law and order, layout, lenity, lineup,
   lucid stillness, lull, marmoreal repose, marshaling, meden agan,
   mental blankness, mental composure, middle way, mildness,
   moderateness, moderation, moderationism, mum, neutrality, nirvana,
   noiselessness, nonviolence, nothing in excess, oblivion, order,
   orderliness, organization, pacifism, passivity, peace,
   peace and quiet, peace of mind, peacefulness,
   philosophic composure, philosophicalness, philosophy, placidity,
   placidness, proportion, prudence, quiescence, quiescency, quiet,
   quiet life, quiet mind, quietism, quietness, quietude, regularity,
   relaxation, repose, rest, restfulness, restraint, routine,
   sangfroid, satori, self-abnegation, self-control, self-denial,
   self-restraint, serenity, setup, silence, silentness,
   silken repose, sleep, slumber, sobriety, solemn silence,
   soothingness, soundlessness, stability, steadiness, still,
   stillness, structure, symmetry, system, tabula rasa, tacitness,
   taciturnity, temperance, temperateness, thoughtfreeness,
   thoughtlessness, tomblike silence, unbuttoned ease,
   unexcessiveness, unextravagance, unextremeness, uniformity,
   unintelligence, unruffledness, vacancy, vacuity, via media, whisht,
   wise passiveness