rydberg constant vs rydberg unit

rydberg constant and rydberg unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
rydberg constant	Yes	No	No	No
rydberg unit	Yes	No	No	No

As nouns, rydberg constant and rydberg unit are synonyms defined as:

rydberg constant and rydberg unit: a wave number characteristic of the wave spectrum of each element

Other synonyms of rydberg constant include rydberg.

rydberg constant (noun)	rydberg unit (noun)
a wave number characteristic of the wave spectrum of each element	a wave number characteristic of the wave spectrum of each element