What is nuclear winter?

A nuclear winter is a climactic condition caused by excessive particulate matter being suspended in the atmosphere that serves to reflect much of the sun's thermal radiation from the ground. This causes a cooling trend that would result in significant temperature drops over broad regions of the planet. By the sun's light being blocked, plant life would also suffer due to insufficient visible and UV radiation to carry out photosynthesis. Since photosynthetic organisms are the base of much of the food chain on land and upper oceans, ecosystems could collapse. Only chemotrophic organisms deep in the oceans and underground and the life that feeds on them would likely survive a severe nuclear winter.