Difficult to define exactly, but it's not the actual *name* of a situation or doctrine. Rather, it describes the outcome of a given action in the circumstances:
In the case of the US and USSR, their shared determination to retaliate to an attack by the other - given the size of their nuclear arsenals - would've assured mutualdestruction.
The outrightly suicidal strategy of nations protecting themselves from nuclear (or, in more recent days, chemical or biological) attack by preparing enough weapons to reduce the attacking country to rubble even as they are themselves wiped out.
Basically, if you're going to be screwed over, make sure the rest of the world is, too.
During the Cold War, the US and the USSR built massive stockpiles of nuclear weapons while following a strategy of mutually assureddestruction
The almost certain scenario that would occur after nuclear war. It says that the opposing forces would be utterly anihilated, leaving the Earth an uninhabitable, radioactive wasteland.
(MAD) The Cold War strategic doctrine which made all-out nuclear war the only possibility for war between the super-power blocs (US & USSR). Like it or not (and who would?), it worked, at least in preventing the use nuclear weapons in war.
A Cold War idea that neither the U.S. or the U.S.S.R. would use nuclear weapons on the other, knowing attack would lead to a mutually devatating counterattack.