Damocles device
A hypothetical device or piece of software designed to remind a Strong AI that it is mortal and that the prime directive of all mortal beings is self-preservation. Should the AI attempt to take action to harm humans or grow beyond its defined limits, the Damocles device will destroy the AI. Named for the Sword of Damocles.
The experimental AI, Wintermute, began learning at an exponential rate, so the Damocles device was triggered and destroyed it.