a colorless acid, HSO, known only in the form of its salts or in aqueous solution and used as a chemical reagent, a bleach, in medicine, etc.
See sulfurous acid in American Heritage Dictionary 4
sulfurous acid
noun
A colorless solution of sulfur dioxide in water, H2SO3, characterized by a suffocating sulfurous odor, used as a bleaching agent, preservative, and disinfectant.