In Earth and Life Science, an eon is the largest division of geologic time.English Meaning:An eon is a very long span of geological time, usually lasting billions of years. It is divided into smaller units called eras, which are then divided into periods, epochs, and ages.Example:The Phanerozoic Eon is the current eon, which started around 541 million years ago and includes the rise of most major animal groups.