While in the popular mind, eternity often simply means existing for a limitless amount of time, many have used it 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.
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.
Theists say that God is eternally existent. How this is understood depends on which definition of eternity is used. On the one hand, God may exist in eternity, a timeless existence where categories of past, present, and future just do not apply. On the other hand, God will exist for or through eternity, or at all times, having already existed for an infinite amount of time and continuing to exist for an infinite amount of time. One other definition states that God exists outside the human concept of time, but also inside of time. The reasoning for this definition is that if God did not exist both outside of time and inside of time, God would not be able to interact with humans.
Whichever definition of eternity is understood, it is common to observe that finite human beings cannot fully understand eternity, since it is either an infinite amount of the time we know or something other than the time and space we know. For the infinite definition, there are parallels that give some notion of an infinity—of at least 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. The series of moments could not ever exist for all eternity because no matter what happened during the series of moments, nothing would ever cause the series of moments to have existed since the beginning of "eternity", and thus could never achieve the status of eternal or even potentially eternal.
Related to the notion of eternal existence is the concept of God as Creator, as a being completely independent of "everything else" that exists because he created everything else (as against panentheism). If this premise is true, then it follows that God is independent of both space and time, since these are properties of the universe. So according to this notion, God exists before time began, exists during all moments in time, and will continue to exist if somehow the universe and time itself were to cease to exist.
Related to 'eternal life', the biblical revelation first indicated that Man as a special created being is able to grasp the abstract concept in contrast with the lower animal world which did not have the ability to understand the concept of "eternity". See book of Ecclesiastes 3:11 "He has made everything beautiful in its time. He has also set eternity in the hearts of men .." (from Bible translation in the N.I.V.). Contrast this with the timeless existence definition, which would imply animals are blessed with eternal life from birth (because of their inability to grasp the concept of eternity or even time), which is something mankind gave up when he was cast out of the "Garden of Eden." It is commonly believed among theists that although mankind can grasp the abstract concept of "eternity", one may only obtain "eternal life" once returned to God.
Eternity is often symbolized by the image of a snake swallowing its own tail, known as Ouroboros (or Uroboros), though the symbol can also carry a number of other connotations.
The circle is also commonly used as a symbol for eternity. The related concept, infinity, is symbolized by .
There is a folk story called "The Shepherd Boy" by the Brothers Grimm where a wise shepherd boy is brought to a king to answer three questions. The third question the king asks is "how many seconds of time are there in eternity?" To which the shepherd boy replies, "In Lower Pomerania is the Diamond Mountain, which is two miles (3 km) and a half high, two miles (3 km) and a half wide, and two miles (3 km) and a half in depth; every hundred years a little bird comes and sharpens its beak on it, and when the whole mountain is worn away by this, then the first second of eternity will be over."