An allegorical painting of a woman, representing eternity. She holds an hourglass, a skull rests on the table beside her, and an Ouroboros floats above her head. All of these are common symbols of eternity.
Eternity in common parlance is an infinitely long period of time. In classical philosophy, however, eternity is defined as what exists outside time while sempiternity is the concept that corresponds to the colloquial definition of eternity.
Eternity 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.
Eternity is often symbolized by the image of a snake swallowing its own tail, known as the Ouroboros (or Uroboros). The circle is also commonly used as a symbol for eternity, as is the mathematical symbol of infinity, . Symbolically, it suggests that Eternity has no beginning or end.