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a long and distinct period of history with a particular feature or characteristic:

(重要な出来事や人物により特徴づけられる)時代, 時期, 年代; (歴史上の)…時代

e.g. his death marked the end of an era

e.g. the era of glasnost.

a system of chronology dating from a particular noteworthy event:

紀元; 記年法〘ある重要な出来事を起点として年を数える方法〙

e.g. the dawn of the Christian era.

Geology a major division of time that is a subdivision of an eon and is itself subdivided into periods:

〘地〙 …代 (!上位年代区分はeon, ｟英｠ aeon(累代); 下位年代区分はperiod(紀))

e.g. the Mesozoic era.

archaic a date or event marking the beginning of a new and distinct period of time.

mid 17th century: from late Latin aera, denoting a number used as a basis of reckoning, an epoch from which time is reckoned, plural of aes, aer- ‘money, counter’.