扇形 is a noun that can be used as an attributive (attr.) to describe something shaped like a fan — it covers both the everyday geometric shape (a fan shape) and the mathematical term 'sector' (the part of a circle between two radii and the arc). In everyday Chinese you'll see it in 扇形窗户 ('fan-shaped window'), 扇形区域 ('sector area'), or in math contexts like 扇形面积 ('sector area').