(1):
(n.) A period of time reckoned from some particular date or epoch; a succession of years dating from some important event; as, the era of Alexander; the era of Christ, or the Christian era (see under Christian).
(2):
(n.) A fixed point of time, usually an epoch, from which a series of years is reckoned.
(3):
(n.) A period of time in which a new order of things prevails; a signal stage of history; an epoch.