chili pepper vs chilli

chili pepper

noun
  • Any fruit of a plant of the botanical genus Capsicum, which has a spicy/burning flavour because it contains capsaicin. (Not all chili peppers are particularly spicy, but all true chili peppers contain some capsaicin, in contrast to e.g. the bell pepper, which is not a true chili pepper and does not.) 

chilli

How often have the words chili pepper and chilli occurred in a corpus of books? (source: Google Ngram Viewer )