A medium in which small, roughly cubic pieces of colored material (usually stone or glass) are embedded in mortar to create patterns or images. The small pieces are called tesserae. Video here.  See also: tile mosaic, pebble mosaic, emblema, opus vermiculatum.

Fragmentary Roman mosaic from Zeugma, Turkey. 1st century BCE.  Zeugma Mosaic Museum, Gaziantep, Turkey.