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.
If you think of the DNS as a tree-structure with each node having its own label, a fully qualified domain name for a specific node would be its label followed by the labels of all the other nodes between it and the root of the tree.