Semicircular is the adjective: shaped like half a circle. A semicircular window, a semicircular drive in front of a house, the semicircular canals in the inner ear (three tiny curved tubes that help you keep your balance). It describes a shape; it is not itself the shape sitting on the page — that thing is a semicircle. Fluent speakers mix the two, and in conversation it rarely matters, but in a geometry exercise the noun and the adjective have different jobs. Though actually, many “semicircular” things in real life are only roughly that shape — a D, a horseshoe, a fan — and nobody comes round with a compass to check.