noun
: a hygroscopic crystalline salt Na 2 S 2 O 3 that commonly crystallizes with five molecules of water, that is usually made by the reaction of sodium sulfite solution with sulfur or of sodium sulfide with sulfur dioxide or in the recovery of sulfur from waste products (as spent oxide), and that is used as a fixing agent in photography, as a reducing agent, bleaching agent, and antichlor, in chemical analysis for the titration of iodine, and in medicine as an antidote in poisoning by cyanides or iodine — called also hypo, sodium hyposulfite