n. 半圆形

[ noun ]
a plane figure with the shape of half a circle
<noun.shape>

Semicircle \Sem"i*cir`cle\, n.
1.
(a) The half of a circle; the part of a circle bounded by
its diameter and half of its circumference.
(b) A semicircumference.

2. A body in the form of half of a circle, or half of a
circumference.

3. An instrument for measuring angles.