a very long span of geological time; in formal usage, a portion of geological time of the greatest magnitude. Three eras are recognized: Paleozoic Era, Mesozoic Era, and Cenozoic Era. Because of the difficulties involved in establishing accurate chronologies, the Precambrian, or earliest, eras are classified independently. An era is composed of one or more geological periods (period). The stratigraphic, or rock, term that corresponds to “era” is “erathem.”