The Collaborative International Dictionary of English v.0.48:

Nirvana \Nir*va"na\, n. [Skr. nirv[=a][.n]a.]
   In the Buddhist system of religion, the final emancipation of
   the soul from transmigration, and consequently a beatific
   enfrachisement from the evils of worldly existence, as by
   annihilation or absorption into the divine. See Buddhism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nirvana
    n 1: (Hinduism and Buddhism) the beatitude that transcends the
         cycle of reincarnation; characterized by the extinction of
         desire and suffering and individual consciousness [syn:
         nirvana, enlightenment]
    2: any place of complete bliss and delight and peace [syn:
       Eden, paradise, nirvana, heaven, promised land,
       Shangri-la]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "nirvana":
   Buddha-field, Canaan, Civitas Dei, KO, Lethe, New Jerusalem, Zion,
   absentmindedness, anorexia, anorexia nervosa, ataraxia, ataraxy,
   blackout, blank mind, blankmindedness, blankness, bliss,
   calm of mind, calmness, catalepsy, catatonia, catatony, coma,
   composure, contemplation, decay of memory, desirelessness,
   devachan, devaloka, elysium, emptiness of mind, empty-headedness,
   empyrean, faint, fallow mind, fatuity, foolishness, forgetfulness,
   forgetting, forgiveness, grayout, happy hunting ground,
   hazy recollection, heedlessness, imperturbability, inanity,
   inappetence, kamaloka, kamavachara, kayo, knockout,
   lack of appetite, lipothymia, lipothymy, lovelessness,
   lucid stillness, marmoreal repose, mental blankness, nepenthe,
   nirvana principle, nothingness, obliteration, oblivion,
   obliviousness, paradise, passionlessness, passivity, peace,
   peacefulness, placidity, placidness, quiescence, quiescency, quiet,
   quietism, quietness, quietude, repose, rest, restfulness, satori,
   semiconsciousness, senselessness, serenity, short memory, silence,
   silken repose, sleep, slumber, stillness, stupor, swoon, syncope,
   tabula rasa, thoughtfreeness, thoughtlessness, tranquillity,
   unambitiousness, unconsciousness, undesirousness, uneagerness,
   unintelligence, unmindfulness, vacancy, vacuity,
   waters of oblivion, wise passiveness



The Devil's Dictionary (1881-1906):

NIRVANA, n.  In the Buddhist religion, a state of pleasurable
annihilation awarded to the wise, particularly to those wise enough to
understand it.