Thereafter means from that point onward, or after that time. When something happens thereafter, it happens in all the time that follows a particular moment or event.

If a library announces that it will close at 5 PM on Fridays and remain closed thereafter until Monday morning, it means the library stays closed from 5 PM Friday through the entire weekend. If a new rule takes effect on September 1st, you must follow it on that day and thereafter, meaning September 1st and every day after.

The word often appears in formal writing, like rules, contracts, or historical accounts. You might read that “The family moved to Boston in 1995 and lived there thereafter” or “Students must arrive by 8 AM; anyone arriving thereafter will be marked tardy.”

Thereafter is similar to “afterward” or “from then on,” but it sounds more official and precise. While you might tell a friend “We ate lunch, and afterward we played soccer,” you'd more likely see thereafter in a school handbook: “The new dress code begins Monday and applies thereafter.” The word helps mark a clear dividing line in time, showing exactly when things change and stay changed.