\begin{definition}[Definition:Orthogonal Trajectories]
Let $\map f {x, y, c}$ define a one-parameter family of curves $F$.
Let $\map g {x, y, c}$ also define a one-parameter family of curves $G$, with the property that:
:Every curve in $F$ is orthogonal to every curve in $G$.
Then $F$ is a '''family of (reciprocal) orthogonal trajectories''' of $G$, and contrariwise.
\end{definition}