\begin{definition}[Definition:Euclid's Definitions - Book XI/21 - Cylinder]
{{EuclidSaid}}
:''When, one side of those about the right angle in a rectangular parallelogram remaining fixed, the parallelogram is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a '''cylinder'''.''
{{EuclidDefRef|XI|21|Cylinder}}
\end{definition}