圆幂定理是平面几何中的一个定理。

一点P对半径R的圆O的幂定义如下：

所以圆内的点的幂为负数，圆外的点的幂为正数，圆上的点的幂为零。

In elementary plane geometry, the power of a point is a real number h that reflects the relative distance of a given point from a given circle. Specifically, the power of a point P with respect to a circle O of radius r is defined by (Figure 1)

where s is the distance between P and the center O of the circle. By this definition, points inside the circle have negative power, points outside have positive power, and points on the circle have zero power. For external points, the power equals the square of the length of a tangent from the point to the circle. The power of a point is also known as the point's circle power or the power of a circle with respect to the point.