conjugate point

1 :  either of two points lying with respect to a conic each on the polar of the other

2a :  one of two points in reference to a rigid body such that if an impulse is applied at either the body will start to rotate about an instantaneous axis through the other, one point becoming the center of suspension of the body considered as a pendulum and the other becoming the center of oscillation b :  one of two points in an optical system either of which is the real or virtual focus of the other