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]

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.