Hot corner
A screen corner that triggers something when the pointer is driven into it — an overview, a screensaver, a window switcher — trading on the fact that a corner is the one target on screen you cannot miss.
The idea rests on Fitts's law: a target at the screen edge is effectively infinite in one direction, and a corner in two. The pointer stops there however hard you throw the mouse. The After Dark screensaver of 1989 exploited this first, with one corner to start the saver immediately and one to hold it off, and Mac OS X made it a system feature in 2003 when Exposé in Panther shipped "Active Screen Corners", letting each corner be bound to the window overview, the desktop or the screensaver. On the X side, the corner was for a long time a special case of edge flipping, the edge-triggered desktop switch in fvwm and its relatives, and Compiz turned corners into triggers for its effects.
Two environments bet their whole interface on it and received different verdicts. GNOME 3 (2011) put a single hot corner in the top left that opened the Activities overview, with no button to press, and spent years being told that users opened the overview every time they went to close a window; a setting to turn it off eventually arrived. Windows 8 (2012) hid the Charms bar, Start and the app switcher in four corners without a single pixel to indicate it, and Windows 10 removed the lot. KDE Plasma has since filed corners under "Screen Edges" with an adjustable delay, which is the compromise most environments have settled on: the feature exists, but someone has to go looking for it.