What is the condition called when there is a reflection from a plane mirror if the angle of incidence is zero?

Hello!

The angle of incidence is usually measured between an incident ray and the normal to a reflecting surface at the point of incidence. The point of incidence is the point where a ray of light strikes a reflecting surface. The term "normal" means a straight line perpendicular to the surface at a given point.

If the angle of incidence is zero, then the incident ray and the corresponding normal coincide. By the main law of reflection, an angle of incidence and the corresponding angle of reflection are congruent, so in such a situation the reflected ray coincides with the normal and with the incident ray.

There is no special name or term for such a situation. We may say that an incident ray is perpendicular to the plane mirror (for a plane mirror, all normals are parallel to each other).