The Collaborative International Dictionary
Protosulphide
Protosulphide \Pro`to*sul"phide\, n. [Proto- + sulphide.] (Chem.) That one of a series of sulphides of any element which has the lowest proportion of sulphur; a sulphide with but one atom of sulphur in the molecule.
Wiktionary
protosulphide
n. (context chemistry English) That one of a series of sulphides of any element which has the lowest proportion of sulphur; a sulphide with only one atom of sulphur in the molecule.