"Era" is a word used in English since 1615, derived from Late Latin ''æra, era'' "an era or epoch from which time is reckoned," probably identical with Latin ''æra'' "counters used for calculation," plural of ''æs'' "brass, money".
The Latin word's use in chronology seems to have begun in 5th century Visigothic Spain, where it appears in the ''History'' of Isidore of Seville, and in later texts. The Spanish era is calculated from 38 BC, perhaps because of a tax (cfr. indiction) levied in that year, or due to a miscalculation of the Battle of Actium, which occurred in 31 BC.
Like epoch, "era" in English it originally meant "the starting point of an age;" the meaning "system of chronological notation" is c.1646; that of "historical period" is 1741.
In chronology, an "era" is the highest level for the organization of the measurement of time. A "calendar era" span of many years which are numbered beginning at a specific epoch, which often marks the origin of a state or cosmology, the birth of a leader, or another significant historical or mythological event; it can be called after either accordingly.