A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor operations, this type of shutdown is often referred to as a "scram" at boiling water reactors and a "reactor trip" at pressurized water reactors. In many cases, a scram is part of the routine shutdown procedure which serves to test the emergency shutdown system.

There is no definitive origin for the term. United States Nuclear Regulatory Commission historian Tom Wellock notes that scram is English-language slang for leaving quickly and urgently (as in scrambling to get away), and he cites this as the original and most likely accurate basis for the use of scram in the technical context.

Scram is sometimes cited as being an acronym for safety control rod axe man or safety cut rope axe man. This was supposedly coined by Enrico Fermi when he oversaw the construction of the world's first nuclear reactor. The core, which was built under the spectator seating at the University of Chicago's Stagg Field, had an actual control rod tied to a rope with a man with an axe standing next to it; cutting the rope would mean the rods would fall by gravity into the reactor core, shutting the reactor down. The axe man at the first chain reaction was Norman Hilberry. In a letter to Raymond Murray (January 21, 1981), Hilberry wrote:

When I showed up on the balcony on that December 2, 1942 afternoon, I was ushered to the balcony rail, handed a well sharpened fireman's axe and told, "If the safety rods fail to operate, cut that manila rope." The safety rods, needless to say, worked, the rope was not cut... I don't believe I have ever felt quite as foolish as I did then. ...I did not get the SCRAM [Safety Control Rod Axe Man] story until many years after the fact. Then one day one of my fellows who had been on Zinn's construction crew called me Mr. Scram. I asked him, "How come?" And then the story.

Leona Marshall Libby, who was present that day at the Chicago Pile, recalled that the term was coined by Volney Wilson who led the team that designed the control rod circuitry:

The safety rods were coated with cadmium foil, and this metal absorbed so many neutrons that the chain reaction was stopped. Volney Wilson called these "scram" rods. He said that the pile had "scrammed," the rods had "scrammed" into the pile.

Other witnesses that day agreed with Libby's crediting "scram" to Wilson. Wellock wrote that Warren Nyer, a student who worked on assembling the pile, also attributed the word to Wilson: "The word arose in a discussion Dr. Wilson, who was head of the instrumentation and controls group, was having with several members of his group," Nyer wrote. "The group had decided to have a big button to push to drive in both the control rods and the safety rod. What to label it? 'What do we do after we punch the button?,' someone asked. 'Scram out of here!,' Wilson said. Bill Overbeck, another member of that group said, 'OK I'll label it SCRAM.'"

The earliest references to "scram" among the Chicago Pile team were also associated with Wilson's shutdown circuitry and not Hilberry. In a 1952 U.S. Atomic Energy Commission (AEC) report by Fermi, the AEC declassified information on the Chicago Pile. The report includes a section written by Wilson's team shortly after the Chicago Pile achieved a self-sustaining chain reaction on December 2, 1942. It includes a wiring schematic of the rod control circuitry with a clearly labeled "SCRAM" line (see image on the right and pages 37 and 48).