A kill switch, also known as an emergency stop (e-stop) and as an emergency power off (EPO), is a safety mechanism used to shut off machinery in an emergency, when it cannot be shut down in the usual manner. Unlike a normal shut-down switch or shut-down procedure, which shuts down all systems in order and turns off the machine without damage, a kill switch is designed and configured to abort the operation as quickly as possible (even if it damages the equipment) and to be operated simply and quickly (so that even a panicked operator with impaired executive functions or a by-stander can activate it). Kill switches are usually designed to be noticeable, even to an untrained operator or a bystander.
In smartphones, a kill switch is a security feature that allows the phone's owner to remotely render the smartphone inoperable if it is lost or stolen. From 2015 this feature is legally required in California for smartphones.
Most kill switches feature a Mollyguard, a removable, protective barrier against accidental activation (e.g. a plastic cover that must be lifted or glass that must be broken). Kill switches are features of mechanisms whose normal operation or foreseeable misuse might cause injury or death; industrial designers include kill switches because damage to or the destruction of the machinery is less important than preventing workplace injuries and deaths.