What is meant by the refractive index of a medium with respect to air?

Snells law of reflection of light states that:

1. The ratio os sine angle of incidence to sine angle of refraction is constant. i.e. sin i/sin r = canstant
2. Angle of incidence is not equal to angle of refraction.
3. Angle of incidence is equal to angle of emergence.

According to the Snell's law,

The ratio of sine of angle of incidence to the sine of angle of refraction is constant.

sine of angle of incidence(i) / sine of angle of refraction(r) = constant

this constant is called refractive index(n).

n = sin i/sin r

This is the expression for the refractive index of water with respect to the air.