What does FQDN mean?

FQDN means Fully Qualified Domain Name. It is also sometimes referred to as an absolute domain name.

This is a complete domain name, with a host, all domain levels, and the top-level domain.

For example, in www.your-domain.co.uk:

www is the host

your-domain is the third-level domain

co is the second-level domain

uk is the top-level domain

The FQDN specifies exactly where your domain sits within the domain hierarchy. No other domain name can have its exact same information.