Mosaic, moh ZAY ihk, is an art form in which small pieces of colored glass, stone, or other material are set into mortar. The pieces, called tesserae or tessellae, fit together to form a picture. Most mosaics decorate ceilings, floors, and interior walls, but some are used for such exterior surfaces as pavements and outside walls. Since the mid-1500's, mosaics have also been used to decorate furniture and objects of personal adornment as well as for architectural ornament.