Millions of years is a term used to describe a very long period of time. It is often used to describe the age of the Earth, which is estimated to be around 4.5 billion years old. The term 'millions of years' can also be broken down into two shorter terms: EONS (4 letters) and EON (3 letters). An EON is the largest division of geological time and is typically divided into eras. An EON is usually millions or billions of years in length.