rem vs radioactivity unit

rem and radioactivity unit both are nouns.

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

As nouns, radioactivity unit is a hypernym of rem; that is, radioactivity unit is a word with a broader meaning than rem:

rem: (roentgen equivalent man) the dosage of ionizing radiation that will cause the same amount of injury to human tissue as 1 roentgen of X-rays
radioactivity unit: a measure of radioactivity

rem (noun)	radioactivity unit (noun)
(roentgen equivalent man) the dosage of ionizing radiation that will cause the same amount of injury to human tissue as 1 roentgen of X-rays	a measure of radioactivity