In a personal computer with the Windows operating system, Ctrl-Alt-Delete is the combination of keyboard keys that the computer user can press at the same time to terminate an application task or to reboot the operating system (have it shut down and restart itself). In Windows 95 or any later systems, Ctrl-Alt-Delete brings up a window that allows a user to see the status of all currently running programs and to terminate any of them, and also offers the options of shutting down, restarting, and so on (the specific options vary slightly with the particular version of windows). In Windows 95 or 98, if Ctrl-Alt-Delete is pressed a second time or twice in a row quickly, the operating system closes all programs that are running and then restarts.
When the operating system seems to hang suspended while waiting for an application program to continue, a user is also left without a way to regain control of the system. The Ctrl-Alt-Delete key combination allows the user to terminate the "hung" application and, if that doesn't work, to reboot the system. The system can also be restarted using the mouse to select Start-->Turn Off the System-->Restart. However, Ctrl-Alt-Delete sometimes works when the menu option doesn't.
The idea to use this key combination as a way to reboot a system originated with an IBM engineer, David Bradley, in 1980 or 1981. The choice of keys and requiring that they be pressed at the same time was intended to make rebooting the system difficult to do accidentally. The reboot key combination is also sometimes referred to as a "three-fingered salute" (to Microsoft's Bill Gates) or as the "Vulcan nerve pinch" (a Star Trek reference).