The term 'billion years, in geology' can be broken down into five crossword solutions: EON (3 letters), EONS (4 letters), AEON (4 letters), ERAS (4 letters), and AGES (4 letters). EON is the longest unit of geological time, and is divided into ERAS, which are further divided into periods, epochs, and AGES. An EON is typically a billion years or more, and is the largest division of geological time. EONS are divided into smaller units called EONS, which are usually hundreds of millions of years long. AEON is another word for an EON, and is sometimes used to refer to a period of time that is longer than an EON. ERAS are the second largest division of geological time, and are usually tens of millions of years long. AGES are the smallest division of geological time, and are usually millions of years long. In summary, the term 'billion years, in geology' can be broken down into five crossword solutions: EON, EONS, AEON, ERAS, and AGES, which represent the largest to smallest divisions of geological time.