antinormal

A normal that points opposite to convention. For example, in solid geometry it is conventional for normals to point outwards, so an antinormal in solid geometry refers to a normal pointing inwards.

Antagonistic to or causing deviations from that which is considered normal.

In the opposite direction or position from what is normal or canonical.

Perpendicular to a tangent of a curve or tangent plane of a surface but pointing opposite to convention.