refractive index vs index of refraction

refractive index and index of refraction both are nouns.

Word	Noun	Adjective	Verb	Adverb
refractive index	Yes	No	No	No
index of refraction	Yes	No	No	No

As nouns, refractive index and index of refraction are synonyms defined as:

refractive index and index of refraction: the ratio of the velocity of light in a vacuum to that in a medium

refractive index (noun)	index of refraction (noun)
the ratio of the velocity of light in a vacuum to that in a medium	the ratio of the velocity of light in a vacuum to that in a medium