Refraction is simply mean a change in path of light in given media

The phenomenon of change in path of light in going from one medium to another is called refraction of light. Thus the refraction of light is the phenomenon of bending of light from its original path  on entering another medium. the refraction occurs right at the boundary of the two media

Snell's Law

Snell's Law relates the indices of refraction n of the two media to the directions of propagation in terms of the angles to the normal. Snell's law can be derived from Fermat's Principle or from the Fresnel Equations.