Definition:Harmonic Conjugates/Harmonic Range

Definition
Let $AB$ and $PQ$ be line segments on a straight line such that $\tuple {AB, PQ}$ is a harmonic range.

Then $P$ and $Q$ are said to be harmonic conjugates with respect to $A$ and $B$.

Also known as
The two pairs of points $\set {A, B}$ and $\set {P, Q}$ can also be said to be apolar.

Also see

 * Definition:Harmonic Range