Dead man's switch
The term dead man's switch originally referred to a mechanism that would trigger if the operator of machinery were to become incapacitated through some means, such as falling asleep. A good example of dead man's switch is the button you have to hold down to start a lawn trimmer or mower.
With respect to software, a dead man's switch generally refers to an alert sent out due to a lack of activity over a certain period of time. Various software can simulate a dead man's switch on your computer in different ways. One example is the automatic engagement of a lock screen after a period of user inactivity. An example of a longer term dead man's switch is Google's Inactive Account Manager that allows designation of a third party to take control of an inactive account.