FQDN

A fully qualified domain name is the whole thing, spelled out: www.example.com, mail.example.com, example.com. Nothing left to assume. Technically it also ends with a trailing dot for the root, as in www.example.com., which you'll see in zone files and almost nowhere else.

It's mostly a term from server and network administration, where "the hostname" might mean just "mail" and the FQDN means "mail.example.com." When a certificate, a DNS record or a config file asks for an FQDN, it wants the full address.

In domain investing you'll rarely need the term, but you'll see it in RDAP output, DNS tools and registry documentation, and it's worth knowing it just means the complete name.