An Etymological Dictionary of Astronomy and Astrophysics

1. General: A period of time marked by a distinctive character, events, etc.
   

2. A system of chronological notation reckoned from a given date.
   

3. Geology: A subdivision of geologic time that is longer than a period but shorter than an eon. Precambrian, Paleozoic, Mesozoic, and Cenozoic are the eras of the time scale from oldest to youngest.

Etymology (EN): From L.L. æra, era “fixed date, era, epoch from which time is reckoned,” probably identical with L. æra “counters used for calculation,” plural of aes “brass, money,” from PIE *aus- “gold” (cf. Av. aiiah- “metal,”
aiianhaēna- “made of metal;” Skt. áyas- “metal;” O.H.G. ēr “ore;” O.E. ora “ore, unworked metal;” Ger. ehern “brazen”).

Etymology (PE): Dowrân, from Ar. daur “age, time; revolution.”