era
/ˈɪərə/noun
1- a long and distinct period of history with a particular feature or characteristic时代：
his death marked the end of an era
他的去世标志着一个时代的结束
the era of glasnost.
开放的时代。
1.1
- a system of chronology dating from a particular noteworthy event纪元：
the dawn of the Christian era.
基督纪元（或公元）的开始。
1.2
- Geology a major division of time that is a subdivision of an aeon and is itself subdivided into periods【地质】代：
the Mesozoic era.
中生代。
1.3
- archaic a date or event marking the beginning of a new and distinct period of time〈古〉（标志新时期开始的）重要日子, 重大事件。
词源
mid 17th cent.: 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'.