Geologic time unit is a term used to describe the divisions of Earth's history. The four main divisions are EON (3 letters), EPOCH (5 letters), AEON (4 letters), and ERA (3 letters). An EON is the longest division of geologic time, lasting hundreds of millions of years. An EPOCH is a shorter period of time within an EON, lasting millions of years. An AEON is even shorter than an EPOCH, lasting tens of millions of years. Finally, an ERA is the shortest division of geologic time, lasting millions of years.