Eternity in common parlance means infinite time. In classical philosophy, however, it is defined as what exists outside time as describing supranatural beings and forces, whereas sempiternity corresponds to the infinitely temporal, non-metaphoric definitions, as recited in requiem prayers for the dead. Thomas Hobbes and many others in the Age of Enlightenment drew on the classical distinction to put forward metaphysical hypotheses such as "eternity is a permanent Now".
Eternity as infinite duration is an important concept in many religions, where the god or gods are said to endure eternally. Some, such as Aristotle, would say the same about the natural cosmos in regard to both past and future eternal duration, and like the eternal Platonic forms, immutability was considered essential. Today cosmologists, philosophers, and others look to analyses of the concept from across cultures and history. They debate, among other things, whether an absolute concept of eternity has real application such for the fundamental laws of physics such as the arrow of time in entropy.


that the cosmos has no beginning. In Aristotle's Metaphysics, eternity is the unmoved mover, understood as the gradient of total synergy. Boethius defined eternity as "simultaneously full and perfect possession of interminable life".


Eternity is often symbolized by the endless snake, swallowing its own tail, the ouroboros. The circle, band or ring is also commonly used as a symbol for eternity, as is the mathematical symbol of infinity,. Symbolically these are reminders that eternity has no beginning/end.