(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.
SCRAM stands for Safety Control Rod Ax Man. In the early days of nuclear power, boron moderator rods were raised and lowered on ropes. In the event of a runaway chain reaction, a man with an axe would chop the rope and drop the rods into the nuclear pile to stop the reaction.
See also molly-guard, TMRC.