\begin{definition}[Definition:Euclid's Definitions - Book I/20 - Triangles by Sides]
{{EuclidSaid}}
:''Of trilateral figures, an '''equilateral triangle''' is that which has its three sides equal, an '''isosceles triangle''' that which has two of its sides alone equal, and a '''scalene triangle''' that which has its three sides unequal.''
{{EuclidDefRef|I|20|Triangles by Sides}}
\end{definition}