[from the nuclear power industry] An emergency-power-off switch (see Big Red Switch), esp. 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.)
“Scram” was in origin a backronym for “Safety Cut Rope Axe Man” coined by Enrico Fermi himself. The story goes that in the earliest nuclear power experiments the engineers recognized the possibility that the reactor wouldn't behave exactly as predicted by their mathematical models. Accordingly, they made sure that they had mechanisms in place that would rapidly drop the control rods back into the reactor. One mechanism took the form of ‘scram technicians’. These individuals stood next to the ropes or cables that raised and lowered the control rods. Equipped with axes or cable-cutters, these technicians stood ready for the (literal) ‘scram’ command. If necessary, they would cut the cables, and gravity would expeditiously return the control rods to the reactor, thereby averting yet another kind of core dump.
Modern reactor control rods are held in place with claw-like devices, held closed by current. SCRAM switches are circuit breakers that immediately open the circuit to the rod arms, resulting in the rapid insertion and subsequent bottoming of the control rods.