And a semicircle is the plane figure contained by the diameter and the circumference cut off by it. And the center of the semicircle is the same (point) as (the center of) the circle.
Euclid's definition corresponds to today's formulation.
A semicircle is the colored plane figure enclosed by the diameter \(\overline{CB}\) with the center at the point \(A\).
Definitions: 1 2 3
Proofs: 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Propositions: 25 26