ÆRA, 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 notæ or abbreviatures 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 abbreviatures A never stood for annus, unless when preceded by V for vir; 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 æra from æs, the tribute-money wherewith Augustus taxed the world; or that of Scaliger himself, who deduces it likewise from æs, though in a different manner. Æs, 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 æra, came by corruption æra, ærum, in the singular; much as Ostia, Ostiam, the name of a place, from Ostia, the mouths of the Tiber.
The difference between the terms æra and epoch is, that the æras are certain points fixed by some people or nation, and the epochs are points fixed by chronologists and historians. The idea of an æra comprehends also a certain succession of years proceeding from a fixed point of time, and the epoch is that point itself. Thus, the Christian æra began at the epoch of the birth of Jesus Christ.