\begin{definition}[Definition:Cissoid of Diocles]
Let $C$ be a circle of radius $a$ with a distinguished point $O$ on its circumference.
Let $L$ be the tangent to $C$ at the other end of the diameter of $C$ through $O$.
Let $R$ be a point on the circumference of $C$.
Let $OR$ be produced to meet $L$ at $S$.
Let $P$ be the point on $OS$ such that $OP$ = $RS$.
The '''cissoid of Diocles''' is the locus of points $P$ as $R$ travels around the circumference of $C$.
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\end{definition}