what is an eon

An eon is a term used in Earth science to describe the longest periods of time, lasting hundreds of millions to billions of years. In formal usage, eons are the longest portions of geologic time, with eras being the second-longest. Geologic eons are also referred to as "eonothems" or simply "eons". There are three recognized eons: the Phanerozoic Eon, the Proterozoic Eon, and the Archean Eon. Less formally, eon often refers to a span of one billion years. The term can also be used more generally to describe an immeasurably or indefinitely long period of time.