Mosaic, in art, decoration of a surface with designs made up of closely set, usually variously coloured, small pieces of material such as stone, mineral, glass, tile, or shell. Unlike inlay, in which the pieces to be applied are set into a surface that has been hollowed out to receive the design, mosaic pieces are applied onto a surface that has been prepared with an adhesive. Mosaic also differs from inlay in the size of its components. Mosaic pieces are anonymous fractions of the design and rarely have the dimensions of pieces for intarsia work (fitted inlay usually of wood), whose function is often the rendering of a whole portion of a figure or pattern. Once disassembled, a mosaic cannot be reassembled on the basis of the form of its individual pieces.

Technical insight is the key to both the creation and the appreciation of mosaic, and the technical aspects of the art require special emphasis. There are also significant stylistic, religious, and cultural aspects of mosaic, which has played an important role in Western art and has appeared in other cultures. Although mosaic is an art form that appears in widely separated places and at different times in history, in only one place—Byzantium—and at one time—4th to 14th centuries—did it rise to become the leading pictorial art.