A nuclear winter was a planet-wide alteration of the climate, usually as the result of a nuclear war or other major events.
Earth experienced nuclear winters in the 21st century after World War III. (TNG: "Encounter at Farpoint", "A Matter of Time")
In 2368, an asteroid impacted on Penthara IV, which threatened to cause a nuclear winter, but the USS Enterprise-D was able to prevent such a disaster. (TNG: "A Matter of Time")
The Vaadwaur homeworld suffered from a nuclear winter, after massive orbital bombardment, from 1484 until at least 2376. (VOY: "Dragon's Teeth")
Uxal experienced a nuclear winter, after an antimatter missile accident, from the 23rd century until 2378 when the USS Voyager neutralized the antimatter radiation. (VOY: "Friendship One")
Talos IV was said to be devastated by atomic war, though it is unclear if this event included a nuclear winter.