a white, crystalline salt, NaSO, used as an antichlor, as a fixing agent in photography, etc.: popularly called sodium hyposulfite or hypo
See sodium thiosulfate in American Heritage Dictionary 4
noun
A white, translucent crystalline compound, Na2S2O3·5H2O, used as a photographic fixing agent and as a bleach. Also called hypo1, hyposulfite, sodium hyposulfite.