Eons - The largest division of geologic time. There are four eons:
* Hadean: The earliest eon, characterized by the formation of Earth and its early evolution.
* Archean: The eon where the first life forms appeared.
* Proterozoic: The eon where multicellular life evolved.
* Phanerozoic: The eon where complex life forms diversified and evolved.
Eras - Subdivisions of eons. Each eon is divided into several eras. For example, the Phanerozoic Eon is divided into three eras:
* Paleozoic: The "ancient life" era, known for the rise and diversification of marine invertebrates, fish, and the first land plants and animals.
* Mesozoic: The "middle life" era, known for the dominance of dinosaurs and the rise of mammals and flowering plants.
* Cenozoic: The "recent life" era, known for the diversification of mammals, birds, and flowering plants.
Periods - Subdivisions of eras. Each era is divided into several periods. For example, the Mesozoic Era is divided into three periods:
* Triassic
* Jurassic
* Cretaceous
Epochs - Subdivisions of periods. Each period is divided into several epochs. For example, the Cenozoic Era is divided into two periods (Paleogene and Neogene) which are further divided into epochs such as the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene.
Ages - Subdivisions of epochs. Ages are the smallest unit of geologic time.
Therefore, the order from largest to smallest is: Eon > Era > Period > Epoch > Age
