Acronym for Fully Qualified Domain Name.
A fully qualified domain name consists of a host and domain name, right up to and including the top-level domain (TLD).
www.wlug.org.nz is a FQDN. The HostName is www (an alias to 'hoiho'), the third-level domain (you don't hear this mentioned very often) is wlug; the second-level-domain is org and the top-level domain is .nz.
You should define your FQDN in hosts(5) (/etc/hosts). Your HostName is defined in a distro-specific file that is usually /etc/hostname, and is set by hostname(1).
5 pages link to FQDN:
