The terminology for chilli peppers can be very confusing as there are several terms (chile, chili, pepper, capsicum, pimento, ají, chile pepper,... you get the idea) used to describe this spice, but strictly speaking not all of them are accurate.
One of the first things we should perhaps clarify is the correct spelling of this word, over which there has been much debate. The three main spellings are chile, chili and chilli and all of these are accepted versions in dictionaries. The current convention for their usage is as follows:
At lovespicyfood.com we will stick to using 'chilli' as this is the version with which we are most familiar. However, please don't feel the need to be constrained by us - the version you use is entirely your choice. Some suggest that at the very least this could be influenced by the region from which the fruit originates. So will that be a jalapeño chile or a birds eye chilli pepper?
Also in common usage to describe this plant or its fruit are the words 'capsicum', 'pepper' and even 'chilli pepper'.
When they were discovered by the Spanish in 1492, Christopher Columbus believed the red fruit he had sampled were related to the highly-prized peppercorns (or 'black pepper') he sought. These berries were mistakenly called 'peppers' as their taste reminded the explorers of black pepper and, much to the confusion of many, the name has stuck.
Chillies are the fruit of plants from the genus 'Capsicum', which is part of the Solanaceae, or nightshade, family. This family includes the likes of the potato, tomato, aubergine (or eggplant), petunia and tobacco. Plants of the Capsicum genus produce the fiery hot, hollow fruit known as chillies and these get their heat from 'capsaicin', a pungent chemical compound. These were native to South and Central America.
'Pepper' (a shortened form of peppercorn) is the dried fruit of the 'piper nigrum' plant and the heat-causing compound is 'piperine'. Peppercorns originated in South India.
So in reality 'capsicums' and 'peppers' are not the same thing nor are they even closely related. But again over time, these two terms have come to be used interchangeably when referring to chillies.
So whilst not strictly correct, saying chilli peppers makes perfect (really?) sense :-)
Okay let's see if we can help clarify some of those confusing (well to us at least!) words we encounter when we start talking about chillies, or is that chilli peppers.....?
Whilst it would be useful to adopt one universally-accepted convention for these terms, this isn't likely to happen any time soon. Besides, language is continually changing and who knows what these delightful fruits will be called in the future. The best plan would be to learn as many of the names, specific fruit types, groups or terms as possible, so you can understand these global variations. In the meantime though, chilli peppers or chiles, pimento or paprika - you take your pick, just enjoy them!