pimiento vs sweet pepper

pimiento and sweet pepper both are nouns.

Word	Noun	Adjective	Verb	Adverb
pimiento	Yes	No	No	No
sweet pepper	Yes	No	No	No

As nouns, sweet pepper is a hypernym of pimiento; that is, sweet pepper is a word with a broader meaning than pimiento:

pimiento: fully ripened sweet red pepper; usually cooked
sweet pepper: large mild crisp thick-walled capsicum peppers usually bell-shaped or somewhat oblong; commonly used in salads

As nouns, pimiento and sweet pepper are synonyms defined as:

pimiento and sweet pepper: plant bearing large mild thick-walled usually bell-shaped fruits; the principal salad peppers

Other synonyms of pimiento include bell pepper, Capsicum annuum grossum, paprika, pimento, sweet pepper plant.

pimiento (noun)	sweet pepper (noun)
fully ripened sweet red pepper; usually cooked	large mild crisp thick-walled capsicum peppers usually bell-shaped or somewhat oblong; commonly used in salads
plant bearing large mild thick-walled usually bell-shaped fruits; the principal salad peppers	plant bearing large mild thick-walled usually bell-shaped fruits; the principal salad peppers