Definition:Positive Unit Normal/Unclosed Surface

Definition
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.

Also see

 * Definition:Positive Unit Normal to Closed Surface