b vs area unit

b and area unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
b	Yes	No	No	No
area unit	Yes	No	No	No

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

b: (physics) a unit of nuclear cross section; the effective circular area that one particle presents to another as a target for an encounter
area unit: a system of units used to measure areas

Other hypernyms of b include square measure.

b (noun)	area unit (noun)
the 2nd letter of the Roman alphabet	a system of units used to measure areas
(physics) a unit of nuclear cross section; the effective circular area that one particle presents to another as a target for an encounter