Eternity often means existence for a limitless amount of time, and may be used to refer to a timeless existence altogether outside of time. There are a number of arguments for eternity, by which proponents of the concept, principally Aristotle, purported to prove that matter, motion, and time must have existed eternally.
The metaphysics of eternity might be summarized by asking if anything can be said to exist outside of or independent of time, and if so how and why? Some consequential metaphysical questions of some importance relate to whether information can be said to exist independently of the human mind, and if so, what would be the content and purpose of such information?
It is an understatement to say that humans cannot fully understand eternity, since it is either an infinite amount of time as we know it or something other than the time and space we know. For the infinite definition, there are parallels that give some notion of a potential infinity, or a series that begins and has not ended. A series of moments that has begun and not ended is, however, not potentially eternal by that definition. A series of moments that has begun and not ended cannot be eternal, because even if it were to continue for the rest of infinite time, there would still be time prior to the initial moment in the series.
Augustine of Hippo wrote that time exists only within the created universe, so that God exists outside of time. For God there is no past or future, but only an eternal present. One need not believe in God in order to hold this concept of eternity. For example, an atheist mathematician can maintain the philosophical tenet that numbers and the relationships among them exist outside of time, and so are in that sense eternal.