Any particular action that occurs. This might sound obvious, but Windows is entirely reliant on responding to events. For example, if you click on the mouse button, this generates a button down event, then a button up event. If you press a key, it generates a key-press event, and so on. Windows waits for events and then responds to them. It won't do something until you tell it to do so. This type of program is described as event-driven.
