A formal joining word meaning "and right after that" or "and because of that". You put it in the middle of a sentence to hook a second event onto the first: "She called his name, whereupon he turned around." Two ideas live inside it, and often both at once. One is timing — the second thing happens immediately, with no gap. The other is cause — the second thing happens because of the first. In most real sentences the two blend, and you do not have to choose. Notice the shape it wants: it joins two full statements, and a comma goes before it. It belongs to careful written English — news reports, history books, court judgments, formal storytelling. In everyday talk people just say "and then" or "so". Older writing also used it for plain "upon which" ("the table whereupon the book lay"), but that use is now rare enough that you can read it and never need to write it.