What counts as a domain?

What counts as a domain?

As a domain, we mean domain.com. Subdomains like: subdomain.domain.com, subdomain1.domain. com are counted as one domain.
Also pages domain.com/pagename, domain.com/pagename2  and all similar structures are counted as a single domain.
However, domain.com, domain.us and domain.pl count as three different domains.