Geologic time is the division of Earth's history into distinct units of time. It is divided into four main units: EON, EPOCH, ERA, and AZOIC. An EON is the largest unit of geologic time, lasting for billions of years. An EPOCH is a smaller unit of time within an EON, lasting for millions of years. An ERA is a smaller unit of time within an EPOCH, lasting for tens of millions of years. Finally, an AZOIC is the smallest unit of geologic time, lasting for thousands of years.