The Collaborative International Dictionary of English v.0.48:

Eternity \E*ter"ni*ty\, n.; pl. Eternities. [F.
   ['e]ternit['e], L. aeternitas, fr. aeternus. See Etern.]
   1. Infinite duration, without beginning in the past or end in
      the future; also, duration without end in the future;
      endless time.
      [1913 Webster]

            The high and lofty One, that inhabiteth eternity.
                                                  --Is. lvii.
                                                  15.
      [1913 Webster]

   2. Condition which begins at death; immortality.
      [1913 Webster]

            Thou know'st 't is common; all that lives must die,
            Passing through nature to eternity.   --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eternity
    n 1: time without end [syn: eternity, infinity]
    2: a state of eternal existence believed in some religions to
       characterize the afterlife [syn: eternity, timelessness,
       timeless existence]
    3: a seemingly endless time interval (waiting)

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "eternity":
   Beulah, Beulah Land, Heaven, Land of Beulah, Paradise,
   a better place, aeon, afterlife, age, ages, all-comprehensiveness,
   all-inclusiveness, an eternity, better world, blue moon,
   boundlessness, ceaselessness, century, coeternity, constancy,
   continualness, continuance, countlessness, datelessness,
   durability, endless time, endlessness, eternal home, eternalness,
   ever-duringness, everlasting moment, everlastingness, everness,
   exhaustlessness, forever, foreverness, glory, happy hunting ground,
   heaven above, heavenly kingdom, high heaven, holiness,
   illimitability, immeasurability, immensity, immortality,
   immutability, incalculability, incessancy, incomprehensibility,
   indestructibility, inexhaustibility, infinite duration,
   infinite goodness, infinite justice, infinite love, infinite mercy,
   infinite power, infinite wisdom, infiniteness, infinitude,
   infinity, innumerability, interminability, kingdom come,
   kingdom of God, kingdom of glory, kingdom of heaven, light,
   limitlessness, long, long time, long while, majesty,
   measurelessness, month of Sundays, never-endingness, neverness,
   no time, numberlessness, omnipotence, omnipotency, omnipresence,
   omniscience, omnisciency, otherworld, perdurability, perenniality,
   perennialness, permanence, perpetualness, perpetuity,
   presence of God, realm of light, right smart spell, sempiternity,
   sovereignty, stability, termlessness, the Promised Land,
   the happy land, the world above, time without end, timelessness,
   ubiquity, unceasingness, unity, universality, unmeasurability,
   world without end, years, years on end