The term domain is just the equivalent short name for a DNS name (ie Domain name).
For more information, check the dedicated page: What is a Domain Name in DNS ?
This examples show how the domain name is used to define a host resource.
The Hostname is a domain name that identifies a host.
Example: host.example.com is a domain name
In the email address label@name, the domain name is the part after the arobase.
The domain portion identifies the point (node) to which the mail should be delivered:
In a url: https://host/path?query#fragment, the domain name is the host part that identifies a HTTP server.