Тhe word "abacus" (abax) means "plate" andit comes from Greek

Terms in Architecture: Abacus Тhe word "abacus" (abax) means "plate" and it comes from Greek. It is used to name the editions of flat and thick plates at the top of a column. Its main function is to ensure stability and wide support of the elements that are above it - most often archives or arches.

The styles in the abacus vary - Egyptian, Greek-Dorian, Romano-Tuscan, and others

Source: styrodesign.eu