Polar

If two points $A$ and $A'$ are Inverse with respect to a Circle (the Inversion Circle), then the straight line through $A'$ which is Perpendicular to the line of the points $AA'$ is called the polar of $A$ with respect to the Circle, and $A$ is called the Pole of the polar.

See also Apollonius' Problem, Inverse Points, Inversion Circle, Polarity, Pole, Trilinear Polar

References

Dörrie, H. 100 Great Problems of Elementary Mathematics: Their History and Solutions. New York: Dover, p. 157, 1965.

Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, pp. 100-106, 1929.