In the Domain Name System (DNS) hierarchy, a second-level domain (SLD or 2LD) is a domain that is directly below a top-level domain (TLD). For example, in example.com, example is the second-level domain of the .com TLD.
Second-level domains commonly refer to the organization that registered the domain name with a domain name registrar. Some domain name registries introduce a second-level hierarchy to a TLD that indicates the type of entity intended to register an SLD under it. For example, in the .uk namespace a college or other academic institution would register under the .ac.uk ccSLD, while companies would register under .co.uk. Some TLDs now break the traditional rule that a TLD is the right most node in a URL. A TLD can have multiple nodes, like pvt.k12.ma.us. A list of the official TLDs can be found at icann.org and iana.org. An ordinal-free term to denote domains under which people can register their own domain name is public suffix domain (PSD).