An "Eon" is a major division of geological time, typically lasting billions of years. It is the largest unit in the geological time scale, which is used by scientists to describe the history of Earth. Eons are further divided into smaller units called eras, which are then divided into periods and epochs. The Earth has four recognized eons: the Hadean, Archean, Proterozoic, and Phanerozoic. Each eon marks significant events in Earth's history, such as the formation of the planet, the emergence of life, and the development of complex organisms. Understanding eons helps scientists study the planet's evolution and the changes in its environment over time.