epoch
/ˈi:pɔk; ?@ ˈepək; `ɛpək/ n
(beginning of a) period of time in history, life, the history of the earth, etc, esp one marked by notable events or characteristics 纪元; 时代; 时期
Einstein's theory marked a new epoch in mathematics. 爱因斯坦理论开创了数学的新纪元.