sulfur trioxide

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

• noun A toxic corrosive liquid or solid, SO₃, that reacts violently with water and produces dense white fumes upon contact with moist air, used as an oxidant and in the sulfonation of organic compounds.

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

• noun inorganic chemistry A corrosive liquid, SO3, which forms sulfuric acid on contact with water; made commercially by the oxidation of sulfur dioxide.