The largest formal unit of geologic time. There are three eons: the Archean, Proterozoic, and Phanerozoic.
The major divisions of the geologic time scale. Eons are divided into intervals know as "eras". Two eons of the geologic time scale are the Phanerozoic (570 million years ago to present) and the Cryptozoic (4,600 million years ago until 570 million years ago).
A time period that consists of eras, periods and epicohs. The longest Eon in earth's geoloic history is the Phanerozoic Eon (See : GEOLOGIC TIME)