Geological time division is a way of categorizing the Earth's history into distinct units. The largest unit is an EON, which is typically hundreds of millions of years long. An AEON is a longer period of time than an EON, and is usually billions of years long. An ERA is a shorter period of time than an EON, and is typically tens to hundreds of millions of years long. Finally, an EPOCH is the shortest unit of geological time, and is usually millions of years long.