Define normal.

The reflected ray is the ray of light that leaves the mirror. A line perpendicular to the mirror's surface can be drawn at the point of incidence where the ray impacts the mirror. This line is called normal. The angle formed by the incident and reflected rays is divided into two equal angles by the normal line.

The angle of incidence is the angle formed by the incident beam and the normal.

The angle of reflection is the angle formed by the reflected beam and the normal.

When a ray of light reflects off a surface, the angle of incidence equals the angle of reflection, according to the law of reflection.

Hence, normal is defined as the perpendicular drawn to the reflecting surface.