noun

• (uncountable) Existence without end, infinite time.
  "Eternity has generally been considered as divisible into two parts; which have been termed, eternity a parte ante, and eternity a parte post: that is, in plain English, that eternity which is past, and that eternity which is to come."
• (uncountable, philosophy, theology) Existence outside of time.
  "We sometimes speak of eternity in contradistinction to time; and often say, "through time and into eternity;" and again "from eternity to eternity," which is simply another form of expressing the same idea, and "pass through time into eternity." in other words, time is a short period allotted to man in his probationary state—and we use the word time in contradistinction to the word eternity, merely for the accommodation of man in his finite sphere, that we may comprehend and learn to measure periods."
• (countable) A period of time which extends infinitely far into the future.
  "Every niche was filled by a funeral urn, and by marble shapes that bent down in a pale eternity of sorrow."
• (metaphysical) The remainder of time that elapses after death.
  "When the wind blows not, where, then, is the wind? / Or when thou art not living, where art thou? / What should the wind care for the hours of calm or thou for death? / Thy life is long, Eternity is short. / So short that, shouldst thou die and Eternity should pass, and after the passing of Eternity thou shouldst live again, thou wouldst say: ‘I closed mine eyes but for an instant.’"