A square or tile for use in a mosaic pavement; a tessera.
Mid 19th century. From post-classical Latin abaculus small cube or tile of coloured glass for making ornamental pavements (former reading in e.g. 16th-cent. editions of Pliny (36. 199, as abaculos, accusative plural), where modern editions have ab oculis; also recorded in an undated glossary) from classical Latin abacus + -ulus.