Definition:Pole of Circle

Definition
Let $S$ be a sphere whose center is $O$.

Let $AB$ be a diameter of $S$, such that $A$ and $B$ are the points of intersection of $AB$ with $S$.

Let $C$ be a (small) circle of $S$ embedded in a plane perpendicular to $AB$.


 * Poles-of-Circle.png

The points $A$ and $B$ are the poles of the circle $C$.