\begin{definition}[Definition:Concyclic Points]
A set of $4$ or more points $S = \set {P_1, P_2, \ldots, P_n}$ is '''concyclic''' if they all lie on the circumference of a circle.
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In the above diagram, points $A, B, C, D$ are '''concyclic''', as they all lie on the circumference of the circle centered at $O$.
\end{definition}