sulfur dioxide

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

• noun A colorless, extremely irritating gas or liquid, SO₂, used in many industrial processes, especially in the manufacture of sulfuric acid, and as a preservative, disinfectant, and bleach. It is also a hazardous pollutant resulting from burning fossil fuels and refining sulfide-containing ores.

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

• noun inorganic chemistry A toxic gas, of molecular formula SO2 that is found in the emissions of volcanos and those of burning coal or petroleum; dissolves in water to form sulfurous acid and, in the presence of oxygen, sulfuric acid, and is thus responsible for acid rain.

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

• noun a colorless toxic gas (SO2) that occurs in the gases from volcanoes; used in many manufacturing processes and present in industrial emissions; causes acid rain