\begin{definition}[Definition:Euclid's Definitions - Book XI/18 - Cone]
{{EuclidSaid}}
:''When, one side of those about the right angle in a right-angled triangle remaining fixed, the triangle is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a '''cone'''.<br>And, if the straight line which remains fixed be equal to the remaining side about the right angle which is carried round, the cone will be '''right-angled'''; if less, '''obtuse-angled'''; and if greater, '''acute-angled'''.''
{{EuclidDefRef|XI|18|Cone}}
\end{definition}