A safety device that must be held engaged by pressure from a human in order for something mechanized to operate. So named because if the operator drops dead, the switch releases and the motion stops. A good control system will perform a "Freddy check" to ensure that the switch is operating properly, and hasn't been taped down or otherwise defeated (apparently a guy named Freddy disliked holding the switch).
A deadman may also be termed an enable. Many pyrotechnics systems will not fire without the operator, as well as another person on an Enable switch, thus guaranteeing two sets of eyes watching for issues.