An aeon, also spelled eon, originally meant "life", "vital force" or "being", "generation" or "a period of time". It is a term used to describe any long, indefinite period of time, although it may also refer to a specific period of a billion years in geology, cosmology, and astronomy. In Gnosticism and Manichaeism, aeons are one of the orders of spirits or spheres of being that emanated from the Godhead and are immaterial, hypostatic ideas). They are similar to Judaeo-Christian angels, existing as beings of light and serving as emanations of God). Each Gnostic system explains aeons in its own way, but all agree that aeons increase in number in proportion to their remoteness from the divinity and that lower aeons share proportionately less in divine energy. At a certain level of remoteness, the possibility of error was said to invade the activity of aeons; in most systems, such error was responsible for the creation of the material universe.