E-Stop

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Emergency Stop. A safety switch that will, when actuated, overrides all other control signals to bring a system to a stop . Sometimes called a "mushroom button" due a typical shape. Many systems include more than one, ideally at convenient locations. An E-Stop should bring the system to a stop as rapidly as possible without creating additional hazard.

NFPA 79: Electrical Standard for Industrial Machinery is helpful in understanding the specific requirements for an Emergency Stop:

9.2.2 Stop Functions. The three categories of stop functions shall be as follows:

(1) Category 0 is an uncontrolled stop by immediately removing power to the machine actuators.
(2) Category 1 is a controlled stop with power to the machine actuators available to acheive the stop then remove power when the stop is acheived.
(3) Category 2 is a controlled stop with power left available to the machine actuators.

9.2.5 Operation.

[...]

9.2.5.3 Stop.

9.2.5.3.1 Each machine shall be equipped with a Category 0 stop.

9.2.5.3.2 [...] Category 0 and Category 1 stops shall be operational regardless of operating modes, and Category 0 shall take priority. [...]

9.2.5.3.3 [...] The reset of the stop function shall not initiate any hazardous conditions.

9.2.5.4 Emergency operation requirements are as follows:
[...]
(2) Once active operation of an emergency stop or emergency switching off has ceased following a[n emergency stop or emergency switching off] command, the effect of this command shall be sustained until it is reset. This reset shall be possible only at that location where the command has been initiated. The reset of the command shall not restart the machinery but only permit restarting.

[...]

9.2.5.4.1.1 In addition to the requirements for stop, the emergency stop shall have the following requirements:

(1) It shall overide all other functions and operations in all modes
(2) Power to the machine actuators, which causes a hazardous condition(s), shall be removed as quickly as possible without creating other hazards (e.g., by the provision of mechanical means of stopping requiring no external power, by reverse current braking for a Category 1 stop).
(3) The reset of the command shall not restart the machine but only permit restarting

9.2.5.4.1.3 The emergency stop shall function as either a Category 0 or a Category 1 stop (see 9.2.2). The choice of the category of the emergency stop shall be determined by the risk assessment of the machine.

Note that as 9.2.5.4.1.3 implies, it may not be prudent to execute an emergency stop by simply cutting power to a system.

The requirement of 9.2.5.4(2) is often met by using buttons that mechanically latch in the active position, and require a positive action to reset (ie, twisting the button cap), such as this example:

Pushbutton Switch, Red, 40 Millimeters Emergency Stop Mushroom Head, Turn to Release