perpole

from The Century Dictionary.

• noun In geometry, as the pole (x₁, y₁) glides along a straight line, the perpolar will not in general turn about a point, but about some curve. But in three cases, when x₁ is constant, when y₁ is constant, when y₁: x₁ is constant, that is, when the pole moves on a straight line parallel to either axis or through the center, the perpolar turns about a point, the perpole of the straight line.