bell pepper vs sweet pepper

bell pepper and sweet pepper both are nouns.

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

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

bell pepper: large bell-shaped sweet pepper in green or red or yellow or orange or black varieties
sweet pepper: large mild crisp thick-walled capsicum peppers usually bell-shaped or somewhat oblong; commonly used in salads

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

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

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

bell pepper (noun)	sweet pepper (noun)
large bell-shaped sweet pepper in green or red or yellow or orange or black varieties	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