Sulfur dioxide: A colorless gas with a pungent odor, SO2 is a significant pollutant released by burning fossil fuels and also an intermediate in the production of sulfuric acid.
Sulfurous acid: $$\text{H}_2\text{SO}_3$$, a weak and unstable acid formed when sulfur dioxide dissolves in water and partially dissociates into bisulfite ions.
Reducing agent: A substance that donates electrons to another substance, thereby reducing its oxidation state; hydrogen sulfite acts as a reducing agent in many reactions.