The Collaborative International Dictionary
Sesquisulphide
Sesquisulphide \Ses`qui*sul"phide\, n. [Sesqui- + sulphide.] (Chem.) A sulphide, analogous to a sesquioxide, containing three atoms of sulphur to two of the other ingredient; -- formerly called also sesquisulphuret; as, orpiment, As2S3 is arsenic sesquisulphide.
Wiktionary
sesquisulphide
n. (context chemistry English) Any sulphide containing three sulphur atoms for every two metal atoms.