The refractive index of a medium
The refractive index above depends on two media. Make the first one a vacuum and it depends on only one: that is the refractive index of the medium.
For example, the refractive index of water is the ratio sin i ÷ sin r for light passing from a vacuum into water.
In practice the refractive index relative to air is used as the refractive index of a medium. The speed of light in air differs from that in a vacuum by very little, and measuring relative to a vacuum is hard to do in practice.
A larger index, a denser medium
Glass (1.5) bends light more than water (1.33): the larger the refractive index, the slower light travels in the medium and the more it bends on entering.