r vs R

r and R both are nouns.

Word	Noun	Adjective	Verb	Adverb
r	Yes	No	No	No
R	Yes	No	No	No

As nouns, r and R are synonyms defined as:

r (noun)	R (noun)
the length of a line segment between the center and circumference of a circle or sphere	the 18th letter of the Roman alphabet
the 18th letter of the Roman alphabet	a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air
	(physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole