\begin{definition}[Definition:Positive Unit Normal/Unclosed Surface]
Let $S$ be a surface in ordinary $3$-space.
Let $S$ be unclosed in space.
A '''positive unit normal''' to $S$ is a unit normal $\mathbf {\hat n}$ to $S$ whose direction is arbitrarily defined, but usually such that it is generally away from the origin of whatever coordinate system is being used.
\end{definition}