The term 'billion-year time period' is often referred to as an EON, which is a three-letter word. An EON is the longest division of geological time and is typically divided into eras. It is estimated that the Earth has been around for 4.5 billion years, so an EON would be a significant portion of that time. The most recent EON is the Phanerozoic EON, which began 541 million years ago and continues to the present day.