1. a period of time marked by distinctive character, events, etc.: an era of progress.
2. the period of time to which anything belongs or is to be assigned.
3. a system of chronological notation reckoned from a given date.
4. a period during which years are numbered and dates reckoned from a particular point of time in the past: the Christian era.
5. a point of time from which succeeding years are numbered, as at the beginning of a system of chronology.
6. a date or an event forming the beginning of any distinctive period.
7. a major division of geological time: Palaeozoic era.
[LL, var. of aera number or epoch by which reckoning is made, era, probably the same word as L aera counters, plural of aes copper, bronze]