epoch

noun

(Gr. epokhê: time interval) One of the intervals of geologic time recommended by the International Subcommission on Stratigraphic Terminology. An epoch is ranked as a third-order time unit, and is the equivalent of the chronostratigraphic unit series. Several epochs form a period; several periods an era. Epochs are themselves subdivided into ages. When used formally, the initial letter is capitalized, e.g. Early Devonian Epoch.