chilli vs chili pepper

chilli and chili pepper both are nouns.

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

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

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

Other synonyms of chilli include chile, chili, chilly.

chilli (noun)	chili pepper (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