A term used to describe the option to host additional domain names on a shared hosting account. With addon domain names you can host multiple domain names/sites with a distinct content on a single hosting account. The addon domain names usually are pointed to some folder on your web space and open a differnt website from your main site. With most web hosting providers you can have separate email accounts for the addon domain. The only downside is that you do not have a separate Control panel for the addon domain. If you need the option to have separate Control Panels for each additional domain name, then you will have to look for some kind of a Reseller package.