\begin{definition}[Definition:Euclid's Definitions - Book I/10 - Right Angle]
{{EuclidSaid}}
:''When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is '''right''', and the straight line standing on the other is called a '''perpendicular''' to that on which it stands.''
{{EuclidDefRef|I|10|Right Angle}}
\end{definition}