Definition:Positive Unit Normal/Closed Surface

Definition
Let $S$ be a surface in ordinary $3$-space.

Let $S$ be closed in space.

A positive unit normal to $S$ is a unit normal $\mathbf {\hat n}$ to $S$ at a point $P$ on $S$ such that the terminal point of $\mathbf {\hat n}$ is on the exterior of $S$.

That is, it is constructed outwards from the enclosed region of space.

Also see

 * Definition:Positive Unit Normal to Unclosed Surface