Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

abacus
     n 1: a tablet placed horizontally on top of the capital of a
          column as an aid in supporting the architrave
     2: a calculator that performs arithmetic functions by manually
        sliding counters on rods or in grooves
     [also: {abaci} (pl)]