1. A manual computing device consisting of a frame holding parallel rods strung with movable counters: Before numbers were even developed as such, counting devices were used to perform everyday calculations; one of these devices was the abacus, which provided merchants accurate data when buying or selling products.
2. A calculator that performs arithmetic functions by manually sliding counters on rods or in grooves: A more recent form of the abacus consists of a frame often made of wood which firmly holds a set of rods or wires with freely sliding beads mounted on them.

The abacus as we know it today doesn't appear as it did when it was originally invented. In ancient times, the abacus was a simple device that was used to count numbers; this only included addition and subtraction.

The newer abaci contain four decks, in which more complex operations can be made; including multiplication and division and they include instructions for determining square roots and cubic roots of numbers.

3. In architecture, a slab or tablet placed horizontally on the top and that filled the space between the columns as an aid in supporting the architrave, the lowest flat stones of structural elements that were on the top of columns: In classical architecture, the shape of the abaci and their edge profiles varied in the different Greek and Roman classical orders.

4. Etymology: a Latin word from the Greek word abax, "counting table". The original abaci were created in sand.