rydberg unit vs rydberg

rydberg unit and rydberg both are nouns.

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

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

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

Other synonyms of rydberg unit include rydberg constant.

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