chili pepper vs chilli

chili pepper and chilli both are nouns.

Word	Noun	Adjective	Verb	Adverb
chili pepper	Yes	No	No	No
chilli	Yes	No	No	No

As nouns, chili pepper and chilli are synonyms defined as:

chili pepper and chilli: very hot and finely tapering pepper of special pungency

Other synonyms of chili pepper include chile, chili, chilly.

chili pepper (noun)	chilli (noun)
very hot and finely tapering pepper of special pungency	very hot and finely tapering pepper of special pungency
plant bearing very hot and finely tapering long peppers; usually red