An eon is composed of several geologic eras, which in turn are composed of geologic periods, which are composed of geologic epochs. We are currently in the Phanerozoic Eon, the Cenozoic Era, the Neogene Period, and the Holocene epoch. More recently, the Hadean, Archean, and Proterozoic "eras" have been considered eons. The geologic timescale in terms of eons, eras, and periods looks like this:
The English word eon is derived from the koine Greek word aion.
Although a proposal was made in 1957 to define an aeon to be a unit of time equal to one billion years (1 Ga), the idea was not approved as a unit of scientific measure and is seldom used for a specific period of time. Its more common usage is for any long, indefinite, period of time. The origin is from the Greek root "aion" for "age" or "life force." A similar Latin word "aevum" for age is still present in words such as Longevity and medieval.