Define the term refractive index of a medium. Can it be less than 1?

The refractive index of the second medium with respect to the first medium is defined as the ratio of the sine of the angle of incidence in the first medium to the sine of the angle of refraction in the second medium.

$μ = \dfrac{\text{Speed of light in vacuum or air(c)}}{\text{Speed of light in that medium (V)}}$

The refractive index of a transparent medium is always greater than 1 (it cannot be less than 1) because the speed of light in any medium is always less than that in vacuum ( i.e. V < c ) and μ = 1 for air or vacuum.