\begin{definition}[Definition:Circle/Semicircle]
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{{EuclidDefinition|book=I|def=18|name=Semicircle}}
In the above diagram, the region bounded by the straight line segment $CD$ and the arc $DBC$ is a '''semicircle'''.
Also, the region bounded by the straight line segment $CD$ and the arc $DFEC$ is a '''semicircle'''.
\end{definition}