\begin{definition}[Definition:Quadrilateral]
A '''quadrilateral''' is a polygon with exactly four sides.
{{EuclidSaid}}
:''{{:Definition:Euclid's Definitions - Book I/19 - Rectilineal Figure}}''
{{EuclidDefRefNocat|I|19|Rectilineal Figure}}
Because it is a polygon, it follows that it also has four vertices.
\end{definition}