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