1. Under; beneath; down
2. Less than normal or normally
3. In a lower state of oxidation; in a low and usually the lowest position in a series of compounds

A compound used as a fixing agent in photographic developing; SYN. sodium thiosulphate.
In photography, a term for sodium thiosulfate, discovered 1819 by John Herschel, and used as a fixative for photographic images since 1837.