The term eTLD stands for "effective top-level domain" and is a domain under which domains can be registered by a single organization.
A top level domain (TLD) is the part of the domain name following the final dot: so for example, the top-level domain in
Suppose only domains directly under top-level domains were registrable by single organizations. Then you would know that the following domains all belonged to the same organization:
xyz.org abc.xyz.org def.xyz.org
However, this does not work as a general rule, because many registrars allow organizations to register domains at levels below the top level. This means that, for example,
aber.ac.uk are owned by 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.
The related concept eTLD+1 means an eTLD plus the next part of the domain name. Because eTLDs are registrable, all domains with the same eTLD+1 are owned by the same organization.
For example, all the following are eTLD+1 domains:
This means that all domains under each of these domains belong to the same organization. For example:
news.sussex.ac.uk blog.sussex.ac.uk admissions.sussex.ac.uk