abacus /ˈabəkəs 
/ ▸ noun (plural abacuses)
1 a simple device for calculating, consisting of a frame with rows of wires or grooves along which beads are slid: at these schools children worked on an abacus to learn basic mathematics
some types of abacus have four beads below the bar, and only one above, on each vertical rod.
2 Architecture the flat slab on top of a capital, supporting the architrave. – ORIGIN late Middle English (denoting a board strewn with sand on which to draw figures): from Latin, from Greek abax, abak- ‘slab, drawing board’, of Semitic origin; probably related to Hebrew 'āḇāq ‘dust’.
EnglishOxford Dictionary of English