The angle of incidence for which the angle of refraction is 90$^o$ when a ray of light passes from a denser medium to a rarer medium is defined as the critical angle for the given pair of media.

When a ray of light travels from a denser medium to a rarer medium, it has the possibility of undergoing total internal reflection instead of refraction. The maximum angle that a ray of light can make when going from a denser medium to a rarer medium without undergoing total internal reflection is called a critical angle.