在几何学中，当圆的两条割线在圆上相遇时，就会形成圆周角。

一般来说，圆周角可被视为共用一个端点的两条弦。

圆周角的基本性质记载于《几何原本》第三卷的第20至22号命题。几何原本中的命题为:

1. 圆周角大小为对同弧的圆心角一半
2. 对同弧的圆周角等
3. 共用一弦的两圆周角大小和为180°

In geometry, an inscribed angle is the angle formed in the interior of a circle when two secant lines (or, in a degenerate case, when one secant line and one tangent line of that circle) intersect on the circle. It can also be defined as the angle subtended at a point on the circle by two given points on the circle