radioactivity unit vs R

radioactivity unit and R both are nouns.

Word	Noun	Adjective	Verb	Adverb
radioactivity unit	Yes	No	No	No
R	Yes	No	No	No

As nouns, R is a hyponym of radioactivity unit; that is, R is a word with a more specific, narrower meaning than radioactivity unit:

radioactivity unit: a measure of radioactivity
R: a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air

Other hyponyms of radioactivity unit include millicurie, Ci, curie, gray, Gy, roentgen, rutherford, rem, millirem, mrem, rad, milliroentgen.

radioactivity unit (noun)	R (noun)
a measure of radioactivity	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