A domain is an authority within the internet that controls its own resources. Its "domain name" is a way to address this authority as part of the hierarchy in a URL - usually the most memorable part of it, for instance a brand name.
A fully qualified domain name (FQDN) contains all necessary parts to look up this authority by name unambiguously using the DNS system of the internet.
For example, in "developer.mozilla.org":
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HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4