Why is the ninth month called September?
1716, earlier aera (1610s), from Late Latin aera, era "an era or epoch from which time is reckoned," probably identical with Latin aera "counters used for calculation," plural of aes (genitive aeris) "brass, copper, money" (see ore, also cf. copper).
The Latin word's use in chronology said to have begun in 5c. Spain (where, for some reason unknown to historians, the local era, aera Hispanica, began 38 B.C.E.; some say it was because of a tax levied that year). Like epoch, in English it originally meant "the starting point of an age;" meaning "system of chronological notation" is c.1640s; that of "historical period" is from 1741, e.g. the U.S. Era of Good Feeling (which was anything but) in reference to the Monroe Administration (1817-24), attested from 1817.
Synonym epoch. Webster's Unabridged makes these words almost synonymous, but "era" usually connotes a span of time rather than a point in time.