FQDN
Fully Qualified Domain Name: a domain name written with every label up to the root, like www.example.com.
An FQDN (Fully Qualified Domain Name) is a domain name that specifies its exact location in the DNS hierarchy, from the host label all the way up to the implicit root. `www.example.com.` (with the trailing dot) is the strictly correct form, though the trailing dot is almost always omitted in everyday use. The opposite is a relative or partially qualified name like `www`, which only makes sense in the context of a search domain. FQDNs matter in zone files, TLS certificates, and many config files, where ambiguity about whether a name is fully qualified is a common source of bugs.