noun
1. obsolete : sulfur trioxide
2. : a heavy corrosive high-boiling oily liquid dibasic acid H 2 SO 4 that is colorless when pure, that was made in early times by distilling green vitriol and is now made commercially from sulfur dioxide by oxidation either in the chamber process usually giving an acid of 65 to 78 percent strength or in the contact process involving conversion of the sulfur dioxide to sulfur trioxide on contact with a catalyst (as of platinum or vanadium oxides) followed by absorption of the trioxide in strong sulfuric acid to form acid of 98 to 99 percent strength or oleum, that is a strong acid and oxidizing agent, that combines energetically with water evolving much heat and is consequently a good drying and dehydrating agent, and that is the most widely used acid in industry (as in the manufacture of superphosphate and other fertilizers, chemicals, detergents, pigments and dyes, explosives, rayon, and storage batteries and in petroleum refining and in pickling metals) — called also oil of vitriol ; compare pyrosulfuric acid