principal axis vs optic axis

principal axis and optic axis both are nouns.

Word	Noun	Adjective	Verb	Adverb
principal axis	Yes	No	No	No
optic axis	Yes	No	No	No

As nouns, principal axis and optic axis are synonyms defined as:

principal axis and optic axis: a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted

principal axis (noun)	optic axis (noun)
a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted	a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted
	in a doubly refracting crystal, the line in the direction of which no double refraction occurs