sweet pepper vs pimento

sweet pepper and pimento both are nouns.

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

As nouns, pimento is a hyponym of sweet pepper; that is, pimento is a word with a more specific, narrower meaning than sweet pepper:

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

Other hyponyms of sweet pepper include bell pepper, green pepper, globe pepper, pimiento.

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

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

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

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