The Collaborative International Dictionary of English v.0.48:

nuclear winter \nuclear winter\ n.
   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.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

nuclear winter
    n 1: 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