Answer
$\textbf{u}\cdot\textbf{v} = 0$, where $\textbf{u}$ and $\textbf{v}$ are orthogonal.
Work Step by Step
Orthogonal means that the angle between the vectors is $90^\circ$.
$\textbf{u}\cdot\textbf{v} = |\textbf{u}||\textbf{v}|cos(90^\circ) = 0$, since $cos(90^\circ) =0$
