Question

define refractive index (n)?

Answer 1

Refractive index is defined as the ratio of the velocity of light in vacuum to its velocity in a specified medium.
$n_{21} = n_{2} / n_{1}$

Answer 2

absolute refractive index is defined as a ratio of the speed of light in the vacuum and in selected medium  

relative refractive index is defined as a ratio of speed of light in two different media

When in relative refrective index ,second medium is vacuum then both will be same

Formula n=c/v