This question already has an answer here:
The era of the Tannaim was from 10-220 CE
The era of the Amoraim was from 200 to 500 CE
A couple questions:
1) Who or what flipped the switch to determine that one era was ending and another was beginning?
2) Were the names "Tanna" & "Amora" assigned retroactively (ex: like WWI was called "The Great War" before WWII happened) OR did they know in the moment that the "Tanna" period ended and that the "Amora" period began?
3) Same questions can be asked re: the "Amoraim"/ "Savoraim" divide, and then "Rishonim" period etc- how did all these eras get set?