Definition: The word abacus has meaning in Greek architecture as the name for a slab of marble on top of the capital of a column. The column capital is at the top of the column and so the square block of marble that is the abacus rests right under whatever the column is supporting. There is, of course, another abacus that is used for counting.
Examples:
Not only does the Doric column stand on the bare floor, but it is topped by a plain capital (a convex disk [called an echinus]
and a square block [called an abacus] formed from a single piece of marble) instead of the intricate scroll work of the Ionic or elaborate acanthus leaves of the Corinthian column.
