A major division of geological time is divided into ERAS, aeons, EONS, and epochs. An ERA is the longest division of geological time, lasting millions of years. An AEON is the second longest division of geological time, lasting hundreds of millions of years. An EON is the third longest division of geological time, lasting billions of years. ERAS and EONS are both plural forms of the same word. An EPOCH is the shortest division of geological time, lasting tens of millions of years.