(From the nuclear power industry) An emergency-power-off switch (see Big Red Switch), especially one positioned to be easily hit by evacuating personnel. In general, this is *not* something you frob lightly; these often initiate expensive events (such as Halon dumps) and are installed in a dinosaur pen for use in case of electrical fire or in case some luckless field servoid should put 120 volts across himself while Easter egging.
See also molly-guard, TMRC.