ArtLex
Art Lex: Mat, Matt, or Matte
This site from the ArtLex Art Dictionary contains definitions for "mat," "matt,", and "matte" as they apply to art. The last definition given in the bunch is the one for "matte."
ArtLex
Art Lex: Glossy
This page contains a definition for what "glossy" means as it applies to art. Some examples of glossy objects are listed.