An entry in the DNS.
'www.yahoo.com', 'www.wlug.org.nz', 'www.linux.org' and 'www.nevada.tla' are all examples of domain names.
Note that, in DNS terminology, the terms "domain name", "domain" and "name" are all synonymous. You may sometimes come across a statement that wlug.org.nz is a "domain", and www.wlug.org.nz is a "name" within that "domain". In fact the name www.wlug.org.nz is also a domain, and the domain wlug.org.nz is also a name. It is true that www.wlug.org.nz is a subdomain of wlug.org.nz. A domain can be used as the name of a machine at the same time as it contains subdomains with other names.