double refraction vs refraction

double refraction and refraction both are nouns.

Word	Noun	Adjective	Verb	Adverb
double refraction	Yes	No	No	No
refraction	Yes	No	No	No

As nouns, refraction is a hypernym of double refraction; that is, refraction is a word with a broader meaning than double refraction:

double refraction: splitting a ray into two parallel rays polarized perpendicularly
refraction: the change in direction of a propagating wave (light or sound) when passing from one medium to another

double refraction (noun)	refraction (noun)
splitting a ray into two parallel rays polarized perpendicularly	the amount by which a propagating wave is bent
	the change in direction of a propagating wave (light or sound) when passing from one medium to another