ERA, in Chronology, a fixed point of time from whence any number of years is begun to be counted. It is sometimes also written in ancient authors Era. The origin of the term is contested, though it is generally allowed to have had its rise in Spain. Sepulveda supposes it formed from A. ER. A. the note or abbreviations of the words annus erat Augusti, occasioned by the Spaniards beginning their computation from the time their country came under the dominion of Augustus, or that of receiving the Roman calendar. This opinion, however ingenious, is rejected by Scaliger, not only on account that in the ancient abbreviations A never stood for annus, unless when preceded by V for erit; but that it seems improbable they should put ER for erat, and the letter A, without any discrimination, both for annus and Augustus. Vossius nevertheless favours the conjecture, and judges it at least as probable as either that of Isidore, who derives era from as, the tribute-money wherewith Augustus taxed the world; or that of Scaliger himself, who deduces it likewise from as, though in a different manner. As, he observes, was used among the ancients for an article or item in an account; and hence it came also to stand for a sum or number itself. From the plural era, came the corruption ara, a, aram, in the singular; much as Ostia, a, Ostiam, the name of a place, from Ostia, the mouths of the Tiber.
The difference between the terms era and epoch is, that the eras are certain points fixed by some people or nation, and the epochs are points fixed by chronologists and historians. The idea of an era comprehends also a certain succession of years proceeding from a fixed point of time, and the epoch is that point itself. Thus the Christian era began at the epoch of the birth of Jesus Christ. See CHRONOLOGY.