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]

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



U.S. Gazetteer Places (2000):

Tranquillity, CA -- U.S. Census Designated Place in California
   Population (2000):    813
   Housing Units (2000): 249
   Land area (2000):     0.628300 sq. miles (1.627289 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.628300 sq. miles (1.627289 sq. km)
   FIPS code:            80266
   Located within:       California (CA), FIPS 06
   Location:             36.648199 N, 120.251549 W
   ZIP Codes (1990):     93668
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Tranquillity, CA
    Tranquillity