Epoch, period (3)
I believe the answer is:
era
I believe this clue is a double definition.
'epoch' is the first definition.
(I know that epoch can be written as era)
'period' is the second definition.
(I know that era is a type of period)
(Other definitions for era that I've seen before include "Stage" , "US equality law" , "Special time" , "Major division of geological time" , "Aeon" .)