The Collaborative International Dictionary of English v.0.48:

Monosulphide \Mon`o*sul"phide\, n. [Mono- + sulphide.] (Chem.)
   A sulphide containing one atom of sulphur, and analogous to a
   monoxide; -- contrasted with a polysulphide; as, galena is
   a monosulphide.
   [1913 Webster]