a strong, crystalline acid, HSO, prepared commercially as a heavy, oily, fuming liquid: used in making explosives and dyes, as a sulfating agent, etc.
See pyrosulfuric acid in American Heritage Dictionary 4
(pīˌrō-sŭl-fyo͝orˈĭk)
noun
A heavy, oily, colorless to dark brown liquid, H2S2O7, produced by adding sulfur trioxide to concentrated sulfuric acid and used in petroleum refining and the manufacture of explosives.