Geological span is a term used to describe the length of time that a particular geological event or process has been occurring. It is usually measured in terms of eons, eras, and epochs. An EON is the longest unit of geological time, lasting for billions of years. An ERA is a subdivision of an EON, lasting for hundreds of millions of years. An EPOCH is a subdivision of an ERA, lasting for millions of years. Together, these three units of geological time make up the geological span.