The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

   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.
      [1913 Webster]

   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.
          [1913 Webster]

   4. A board, tray, or table, divided into perforated
      compartments, for holding cups, bottles, or the like; a
      kind of cupboard, buffet, or sideboard.
      [1913 Webster]

   Abacus harmonicus (Mus.), an ancient diagram showing the
      structure and disposition of the keys of an instrument.
      --Crabb.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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