Mosaics are

a. images made with glass that has been colored by metallic salts are pieced together like a collage, and are joined by leading
b. images made by arranging colored pieces on a flat surface, which are held using leading.
c. artwork created by arranging cut out or torn material pasted onto a surface. means "to glue" in french.
d. stones or tiles that are then embedded with a filler to connect the colored pieces.