What is "arbelos"

n. (context geometry English) A plane region bounded by a semicircle of diameter 1, connected to semicircles of diameters ''r'' and (1 − ''r''), all oriented the same way and sharing a common baseline.

In geometry, an arbelos is a plane region bounded by three semicircles connected at the corners, all on the same side of a straight line (the baseline) that contains their diameters.

The earliest known reference to this figure is in the Book of Lemmas, where some of its mathematical properties are stated as Propositions 4 through 8.