1 a system of chronology reckoning from a noteworthy event (the Christian era).
2 a large distinct period of time, esp. regarded historically (the pre-Roman era).
3 a date at which an era begins.
4 Geol. a major division of time.
Etymology: LL aera number expressed in figures (pl. of aes aeris money, treated as fem. sing.)
Useful english dictionary. 2012.