bulla vs pustule

bulla and pustule both are nouns.

Word	Noun	Adjective	Verb	Adverb
bulla	Yes	No	No	No
pustule	Yes	No	No	No

As nouns, pustule is a hyponym of bulla; that is, pustule is a word with a more specific, narrower meaning than bulla:

bulla: (pathology) an elevation of the skin filled with serous fluid
pustule: a small inflamed elevation of skin containing pus; a blister filled with pus

Other hyponyms of bulla include water blister, blood blister.

bulla (noun)	pustule (noun)
the round leaden seal affixed to a papal bull	a small inflamed elevation of skin containing pus; a blister filled with pus
(pathology) an elevation of the skin filled with serous fluid