After that, from then on; thenceforth.
"Thereafter, he dedicated every evening to studying languages until he became fluent."
In plain English: Thereafter means after that specific time or event has happened.
"The company announced its new policy, and thereafter all employees must follow the updated guidelines."
Thereafter comes from Middle English, which borrowed it directly from Old English to mean "after that." The word is simply a combination of the words there and after.