Abacus \Ab"a*cus\ ([a^]b"[.a]*k[u^]s), n.; E. pl. Abacuses; L. pl. Abaci (-s[imac]). [L. abacus, abax, Gr. 'a`bax]

1. A table or tray strewn with sand, anciently used for drawing, calculating, etc. [Obs.]

2. A calculating table or frame; an instrument for performing arithmetical calculations by balls sliding on wires, or counters in grooves, the lowest line representing units, the second line, tens, etc. It is still employed in China.

3. (Arch.)
   (a) The uppermost member or division of the capital of a column, immediately under the architrave. See Column.
   (b) A tablet, panel, or compartment in ornamented or mosaic work.

4. A board, tray, or table, divided into perforated compartments, for holding cups, bottles, or the like; a kind of cupboard, buffet, or sideboard.

Abacus harmonicus (Mus.), an ancient diagram showing the structure and disposition of the keys of an instrument. --Crabb.

Source: Webster's Revised Unabridged Dictionary (1913)