Reflection
Definition
Light bouncing off a surface — the reason you can see yourself in a mirror.
Technical definition
The return of a wave from a boundary without entering the second medium. For specular reflection, the angle of incidence equals the angle of reflection, θ_i = θ_r, both measured from the normal.
Example
A calm lake mirrors the mountains because its surface is smooth on the scale of light's wavelength — each ray reflects at an equal angle, preserving the image. Roughen the water and the same rays scatter: that is diffuse reflection.
Reflection is not refraction. Reflection bounces light back into the same medium (mirror); refraction bends light as it enters a new medium (lens, water). And the law of reflection measures angles from the normal — the perpendicular — not from the surface. Measuring from the surface is the most common ray-diagram error.