what is a eon in years

An eon is a unit of time used in geology and astronomy to describe a very long period of time. In astronomy, an eon refers to one billion years. In geology, an eon is the longest unit of geologic time, roughly one billion years long. The time span of 4.5 billion years is divided into smaller segments or units called eons, eras, periods, epochs, and ages. Geologists have given names to all of the eons, eras, periods, and epochs that they have found by looking at rocks. The names given to each eon describe something about that time, or the rocks that were made at that time.