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a device for making arithmetic calculations, consisting of a frame set with rods on which balls or beads are moved.
Architecture. a slab forming the top of the capital of a column.
Contemporary Examples

Thus, Goldman found them a willing buyer for the junk piled into abacus.
Goldman’s Dirty Customers John Carney April 20, 2010

But abacus and similar deals were already sucking money out of Rhineland, according to a person familiar with the matter.
Goldman’s Dirty Customers John Carney April 20, 2010

Historical Examples

Whenever the dimensions of the column were sufficiently great the stone beams which met upon the die or abacus had oblique joints.
A History of Art in Ancient Egypt, Vol. II (of 2) Georges Perrot

This, as pictured in the text, is the common Gerbert abacus.
The Hindu-Arabic Numerals David Eugene Smith

Now look under the abacus of this capital; you will find the stone hollowed out wonderfully; and also in this arch-mould.
A Laodicean Thomas Hardy

The abacus has a width equivalent to the thickness of the bottom of a column.
Ten Books on Architecture Vitruvius

The more slender the shaft, the greater, proportionally, may be the projection of the abacus.
The Stones of Venice, Volume I (of 3) John Ruskin

The abacus is the crowning member of the capital, as the capital is of the column.
History of Ancient Art Franz von Reber

We worked, not with slate and pencil, but with a rectangular wooden frame set with beads, resembling an abacus.
A Japanese Boy Shigemi Shiukichi

Though the month was February below it was May in the abacus of the column.
Two on a Tower Thomas Hardy

noun (pl) -ci (-ˌsaɪ), -cuses
a counting device that consists of a frame holding rods on which a specific number of beads are free to move. Each rod designates a given denomination, such as units, tens, hundreds, etc, in the decimal system, and each bead represents a digit or a specific number of digits
(architect) the flat upper part of the capital of a column
n.

late 14c., “sand table for drawing, calculating, etc.,” from Latin abacus, from Greek abax (genitive abakos) “counting table,” from Hebrew abaq “dust,” from root a-b-q “to fly off.” Originally a drawing board covered with dust or sand that could be written on to do mathematical equations. Specific reference to a counting frame is 17c. or later.