\begin{definition}[Definition:Pole of Circle]
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 $S$
Let $C$ be a (small) circle of $S$ embedded in a plane perpendicular to $AB$.
 
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The points $A$ and $B$ are the '''poles''' of the circle $C$.
\end{definition}