r vs radius of curvature

r and radius of curvature both are nouns.

Word	Noun	Adjective	Verb	Adverb
r	Yes	No	No	No
radius of curvature	Yes	No	No	No

As nouns, radius of curvature is a hyponym of r; that is, radius of curvature is a word with a more specific, narrower meaning than r:

r: the length of a line segment between the center and circumference of a circle or sphere
radius of curvature: the radius of the circle of curvature; the absolute value of the reciprocal of the curvature of a curve at a given point

Other hyponyms of r include semidiameter.

r (noun)	radius of curvature (noun)
the length of a line segment between the center and circumference of a circle or sphere	the radius of the circle of curvature; the absolute value of the reciprocal of the curvature of a curve at a given point
the 18th letter of the Roman alphabet