“+...- noun
Etymology: International Scientific Vocabulary pyr- + sulfuric
: an unstable, crystalline acid H 2 S 2 O 7 that is usually handled commercially as a thick oily fuming liquid, that is formed by the union of sulfur trioxide with sulfuric acid, that is a chief component of fuming sulfuric acid, and that is converted to sulfuric acid when mixed with water — called also disulfuric acid ; compare oleum