1. A position whereby the United States and the USSR - Union of Soviet Socialist Republics - would in a nuclear war, be able to destroy each other with their arsenals of nuclear weapons. Each maintaining the possibility to strike the other even if one party fired first. The phrase mutually assured destruction thereby implies that both parties assumed the ability to strike and destroy the other. A catastrophic eventuality if it ever arose. Both parties thus maintained this position, assuring each other than nuclear weapons would not be used, even if a conventional war broke out. The downfall of the Soviet system and emergence of the Russian federation did not make a nuclear war ineffectual, rather it maintains that the two parties still have the capability - and underscores the fact that a nuclear war between these nuclear giants is an inevitability.
2. While this term is specifically used to term the USA and Russia, it may also be applied to China as the latter has built an intercontinental ballistic arsenal that would be deployable if the USA and China ever faced off.