noun, noun which may take the genitive case particle 'no'
semicircle; half circle
Refers to a half-circle shape or a semicircular object. Commonly used in geometry, design, and everyday descriptions.
子供たちは半円になって座った。
The children sat in a semicircle.
この建物の窓は半円の形をしている。
The windows of this building are semicircular in shape.
半円 is the standard geometric term for a semicircle, whereas 半月 in this sense is a more descriptive, shape-based word.
Compound of 半 (half) and 円 (circle). The reading is the standard on'yomi combination.