Sentence:  Every name in the DNS tree is a domain, even if it is terminal, that is, has no <WSD>subdomains</WSD>. Every <WSD>subdomain</WSD> is a domain and every domain except the root is also a <WSD>subdomain</WSD>. 
Meaning of the word marked with <WSD> tags:  A domain name that has been prefaced with additional parts separated with periods.