The term 'One Billion Years, Geologically' can be broken down into two crossword solutions: EON (3 letters) and EONS (4 letters). An EON is a very long period of time in geology, usually lasting hundreds of millions of years. It is the largest division of geological time and is divided into eras. An EON is typically composed of several eras. EONS are typically used to describe the age of the Earth, which is estimated to be around 4.5 billion years old. EONS are also used to describe the age of the universe, which is estimated to be around 13.8 billion years old.