What is meant by the refractive index of a material?

What is meant by the refractive index of a material?

It is the ratio of the sine of angle of incident to the sine of angle of refraction for a ray of light traveling from air in to a given medium.

i.e., Refractive index, n =sin⁡i/sin⁡r

OR

This is the ratio of the speed of light in a vacuum to speed of light in a medium.

Refractive index, n =(Speed of light in vacuum, c /(Speed of light in a medium, c_m )