Registrable domain

A registrable domain is a domain that can be (or has already been) registered by an individual or an organization through a domain name registrar. It's the highest level domain for a single site on the web.

Registrable domains are created directly underneath an effective top-level domain (eTLD), such as .org, .com, or .ac.uk. For this reason a registrable domain is sometimes called an "eTLD+1".

For example, all of the following are registrable domains:

crookedtimber.org
theguardian.com
sussex.ac.uk

This means that all domains under each of these domains belong to the same organization. For example:

 film.theguardian.com
music.theguardian.com

      news.sussex.ac.uk
      blog.sussex.ac.uk
admissions.sussex.ac.uk

Note that not all eTLDs are top-level domains, because many registrars allow organizations to register domains at levels below the top level. In the example above, .ac.uk is a subdomain of the top-level .uk domain, so sussex.ac.uk and aber.ac.uk can be registered to different organizations.

Because this is a matter of the registrar's policies, it's impossible to tell algorithmically whether a given domain name suffix (like .ac.uk) is publicly registrable or not. The Public Suffix List is a list of all suffixes under which organizations can directly register names: that is, it is a list of eTLDs.