pyrosulfuric acid

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

• noun A fuming, dense, oily, colorless to dark brown liquid or hygroscopic crystalline solid, H₂S₂O₇, the primary constituent of oleum, produced by adding sulfur trioxide to concentrated sulfuric acid, and used in petroleum refining and in the manufacture of explosives.

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

• noun inorganic chemistry The acid, H2S2O7, formed by the reaction of sulfur trioxide with sulfuric acid; used in the manufacture of explosives etc.