Hitherto means up until now, or up to this point in time. It's a formal way of saying “so far” or “until this moment.”

You might read in a history book that a scientist made a discovery about something hitherto unknown, meaning nobody had understood it before that moment. Or a story might describe how a character faces challenges hitherto unimagined, meaning they had never pictured such difficulties until now.

The word often appears in writing more than in everyday conversation. You're unlikely to tell your friend, “I've been hitherto unable to finish my homework,” but you might encounter it in books, especially older ones. Classic novels and formal documents use hitherto to mark a turning point: the moment when something that was true before stops being true, or when something new begins.

When you see hitherto, pay attention. The writer is usually signaling that things are about to change, and that what was true hitherto is about to shift.