Fully Qualified Domain Name
完全限定域名 （Fully qualified domain name）是域名的一种，能精确标识其在域名系统（DNS）树状图下的一个确定位置。它没有任何曲解的可能，只能通过一种方式解析。
WikiPedia
A fully qualified domain name (FQDN), sometimes also referred to as an absolute domain name,[1] is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone.[2] A fully qualified domain name is distinguished by its lack of ambiguity in terms of DNS zone location in the hierarchy of DNS labels: it can be interpreted only in one way.[1]