Countless centuries is a term used to describe a very long period of time. It can be broken down into three parts: EONS, EON, and AEON. An EON is the longest division of geological time and is typically divided into two eras. An EON is typically millions of years in length. An AEON is a much shorter period of time, usually lasting hundreds or thousands of years. Together, these three terms describe a vast amount of time that is difficult to comprehend.