A feature of more advanced screensavers, such as those found in Idlewild and After Dark, to behave a certain way when the mouse pointer is in one corner of the screen. When the mouse is placed in a "hot corner," it usually signifies the users intent to activate the screen saver immediately. Users may also designate corners (cold corners? I don't know) that suppress the screen saver when the mouse pointer is placed there. This is useful for when you're waiting for a large project to compile or execute but don't want the screen saver there to muck it up.