The Collaborative International Dictionary of English v.0.48:

Quietude \Qui"e*tude\, n. [L. quietudo: cf. F. qui['e]tude.]
   Rest; repose; quiet; tranquillity. --Shelley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quietude
    n 1: a state of peace and quiet [syn: tranquillity,
         tranquility, quietness, quietude]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "quietude":
   Buddha-like composure, Oriental calm, arrangement, array, ataraxia,
   ataraxy, awful silence, brevity, briefness, brusqueness,
   calm disposition, calm of mind, calmness, composure, conciseness,
   concord, contemplation, cool, coolheadedness, coolness,
   countenance, curtness, dead, dead of night, deathlike silence,
   deployment, disposal, disposition, dumbness, easy mind,
   economy of words, formation, golden silence, harmony, hush,
   hush of night, icy calm, imperturbability, imperturbation,
   inaudibility, indisturbance, laconicalness, laconism,
   law and order, layout, lineup, lucid stillness, lull,
   marmoreal repose, marshaling, mental composure, mum, muteness,
   nirvana, noiselessness, obmutescence, order, orderliness,
   organization, pauciloquy, peace, peace and quiet, peace of mind,
   peacefulness, philosophic composure, philosophicalness, philosophy,
   placidity, placidness, proportion, quiescence, quiescency, quiet,
   quiet life, quiet mind, quietism, quietness, regularity, repose,
   rest, restfulness, routine, sangfroid, satori, serenity, setup,
   shortness, silence, silentness, silken repose, sleep, slumber,
   solemn silence, soothingness, soundlessness, sparingness of words,
   speechlessness, still, stillness, structure, symmetry, system,
   tacitness, taciturnity, terseness, tomblike silence, tranquillity,
   uniformity, unloquaciousness, unruffledness, untalkativeness,
   whisht, wise passiveness, wordlessness