An EON Subdivision is a period of time that is divided into two or more eras. It is the longest division of geological time and is typically millions of years in length. The term 'EON' is derived from the Greek word for 'AGE' and is used to describe the largest divisions of geologic time. An example of an EON Subdivision is the Phanerozoic EON, which is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.