The **polar line** of a point $P$ is the line that passes through the inverse $P'$ and is perpendicular to $OPP'$. $P$ is called the **pole** of such line

If $P$ lies outside the inversion circle $\Gamma$, then the polar of $P$ intersects $\Gamma$ in the tangency points of the two tangents to $\Gamma$ thrown from $P$