nuclear winter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A worldwide darkening and cooling of the atmosphere with consequent devastation of life forms, believed to be a probable outcome of large-scale nuclear war.

from the GNU version of the Collaborative International Dictionary of English.

• noun a hypothetical lowering of global temperature postulated as the aftermath of a full nuclear war. The mechanism of such an effect depends primarily on the injection into the atmosphere of small solid particles, as soot from burning forests, which reflect sunlight and cause a reduction in solar energy retained by the earth.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A predicted drop in global temperature following a nuclear war due to dust in the upper atmosphere reducing sunlight reaching the ground.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a long period of darkness and extreme cold that scientists predict would follow a full-scale nuclear war; a layer of dust and smoke in the atmosphere would cover the earth and block the rays of the sun; most living organisms would perish