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).

Semicircular Semicircular is the adjective是形容詞: shaped like half a circle意思是形狀像半個圓。A semicircular window半圓形的窗戶, a semicircular drive in front of a house房子前面的半圓形車道, the semicircular canals in the inner ear內耳的半規管 (three tiny curved tubes that help you keep your balance)（三條幫助你保持平衡的細小彎曲管子）。

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.

It它 describes a shape是用來描述形狀的字; it is not itself它本身不是 the shape sitting on the page紙上畫的那個形狀 — that thing is a semicircle那個東西叫做semicircle（半圓形）。Fluent speakers母語人士 mix the two常把這兩個字混用, and in conversation在對話中 it rarely matters通常沒差, but in a geometry exercise但在幾何練習裡 the noun and the adjective名詞和形容詞 have different jobs功能是不一樣的。

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.

Though actually不過其實, many "semicircular" things in real life現實生活中很多「semicircular」的東西 are only roughly that shape也只是大概那個形狀而已 — a D, a horseshoe, a fan像D字形、馬蹄鐵、扇子 — and nobody comes round with a compass to check沒有人會拿圓規來檢查。