Design Nomenclature / Form & Geometry
Normal
Also called surface normal, face normal, vertex normal, winding order, face orientation
The unit vector perpendicular to a surface at a point, fixing which side is outward. A face normal follows from the winding order of its vertices — counter-clockwise seen from the front, by near-universal convention. A vertex normal is a weighted average of the adjacent face normals, and it, not the face normal, drives smooth shading.
In practice
'Flipped normals' means reversed winding, not a corrupted vector; the fix re-orders vertices. Backface culling, boolean sidedness, draft analysis, and a slicer's inside/outside test all read this one value.