A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately 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 (BWR), a "reactor trip" at pressurized water reactors and at a CANDU reactor. In many cases, a scram is part of the routine shutdown procedure, which serves to test the emergency shutdown system.

The etymology of the term is a matter of debate. United States Nuclear Regulatory Commission historian Tom Wellock notes that scram is English-language slang for leaving quickly and urgently, and cites this as the original and most likely accurate basis for the use of scram in the technical context. A persistent alternative explanation posits that scram is an acronym for "safety control rod axe man", which was supposedly coined by Enrico Fermi when the world's first nuclear reactor was built under the spectator seating at the University of Chicago's Stagg Field. That reactor had an actual control rod tied to a rope, with a man with an axe standing next to it. It could also stand for "safety control rods activation mechanism" or "safety control rod actuator mechanism", but both of these are probably backronyms from the original, non-technical usage.

The Russian name, AZ-5 (АЗ-5, in Cyrillic), is an abbreviation for аварийная защита 5-й категории (avariynaya zashchita 5-y kategorii), which translates to "emergency protection of the 5th category" in English.