\begin{definition}[Definition:Quadrilateral/Parallelogram/Altitude]
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An '''altitude''' of a parallelogram is a line drawn perpendicular to its base, through one of its vertices to the side opposite to the base (which is extended if necessary).
In the parallelogram above, line $DE$ is an '''altitude''' of the parallelogram $ABCD$.
The term is also used for the length of such a line.
\end{definition}