From AMS Glossary
The primary and largest division of geologic time.
Limits correspond with major, global crustal events, changes in sea level and/or climate, or biotic changes. Five geologic eras have been established: Archeozoic [before 2500 million years ago (Ma)], Proterozoic (2500 to 570 Ma), Paleozoic (570 Ma to ca. 250 Ma), Mesozoic (ca. 250 to ca. 70 Ma), and Cenozoic (since ca. 70 Ma). All eras are divided into at least two geologic periods.