An add-on domain is a domain that points to a folder within a website, but is not merely a redirect to it, as is a parked domain. For example, supposing a website's URL was www.maindomain.com, you could set up an add-on domain such as www.addondomain.com to point to www.maindomain.com/subdirectory/. Users visiting www.addondomain.com/file.html would actually be seeing www.maindomain.com/subdirectory/file.html, even though 'www.addondomain.com/file.html' would appear as the address in their browser.