Whereupon' is an adverb that means 'immediately after something has happened or been done'. It is often used to describe a situation in which one event follows another in quick succession. For example, 'Whereupon the storm began to rage' would mean that the storm started immediately after something else had happened. It can also be used to describe a situation in which one action leads to another, such as 'Whereupon he opened the door, the room was filled with light'.