Definition of Roussin's salt
: any of two series of alkali metal salts that are nitrosyl and sulfur complexes of iron:
a
: a red unstable salt having the general formula M[Fe(NO)2S] and obtainable by reaction of nitric oxide with ferrous sulfide
b
: a black more stable salt having the general formula M[Fe4(NO)7S3] and obtainable (as by treatment with alkali) from a red salt
