Orthopole
In geometry, the orthopole of a system consisting of a triangle ABC and a line ℓ in the same plane is a point determined as follows. Let  be the feet of perpendiculars dropped on ℓ from  respectively. Let  be the feet of perpendiculars dropped from  to the sides opposite  (respectively) or to those sides' extensions. Then the three lines  are concurrent. The point at which they concur is the orthopole.

Due to their many properties, orthopoles have been the subject of a large literature.
Some key topics are determination of the lines having a given orthopole and orthopolar circles.

Literature
 Orthopole=Ортополюс. In Russian

References

Points defined for a triangle