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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.
Historical Examples

From these were derived the abaci of the Chinese and Russians.
The Races of Man Joseph Deniker

The little birds under the angles of the abaci should not be overlooked.
Byzantine Churches in Constantinople Alexander Van Millingen

Each window has three very slender white marble shafts, with capitals and with abaci moulded on each side.
Portuguese Architecture Walter Crum Watson

The capitals here are very close imitations of Classical work, with the abaci frequently concave on plan.
Brick and Marble in the Middle Ages George Edmund Street

The architrave rests upon them, and, as in Greece and Egypt, its immediate weight is borne by abaci.
A History of Art in Chalda & Assyria, v. 1 Georges Perrot

The columns stand less than their diameter apart, and the abaci are so broad that they nearly touch.
Vacation days in Greece Rufus B. Richardson

The door and window have respectively six and three jamb-shafts, and the abaci throughout are square in plan.
Some Account of Gothic Architecture in Spain George Edmund Street

But the abaci of its capitals run from the jambs across to the buttresses, as is the case with those of the doorway.
Bell’s Cathedrals: Chichester (1901) Hubert C. Corlette

The abaci are all square in plan, and both bases and caps are set at right angles to the direction of the arches they support.
Some Account of Gothic Architecture in Spain George Edmund Street

These abaci, which are finely moulded, are not more than about two and a half inches in depth.
Bell’s Cathedrals: Chichester (1901) Hubert C. Corlette

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.