A domain name that extends all the way back to root. Often written as FQDN.

gomer.gimboid.com. is an FQDN.

A common error is to leave the "." at the end off.

gomer.example.com is NOT an FQDN, by leaving out the last "." Root is not included.

An FQDN is required any time you reference a domain name outside a zone, optional when referencing names inside the zone.

CNAME records, NS records, and MX records might refer to names outside the zone, they often require an FQDN

Note: If you put an unqualified domain name in any resource record, the name of the zone the record is in is appended, complete with the dot, to form a fully qualified domain name.

So if an MX record in exampe.com, pointing to Gomer.example.com does not include the final "." it will be appended as Gomer.example.com.example.com.

However, if the MX record in the example had merely pointed to "Gomer", the auto-append would have created gomer.example.com.