In the Domain Name System

Domain name system

The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...

 (DNS) hierarchy, a second-level domain (SLD) is a domain that is directly below a top-level domain

Top-level domain

A top-level domain is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a...

 (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

Domain name registrar

A domain name registrar is an organization or commercial entity, accredited by both ICANN and generic top-level domain registry to sell gTLDs and/or by a country code top-level domain registry to sell ccTLDs; to manage the reservation of Internet domain names in accordance with the guidelines of...

. Some domain name registries

Domain name registry

A domain name registry is a database of all domain names registered in a top-level domain. A registry operator, also called a network information center , is the part of the Domain Name System of the Internet that keeps the database of domain names, and generates the zone files which convert...

 introduce a second-level hierarchy to a TLD that indicates the type of organization intended to register an SLD under it. For example, in the .uk

.uk

.uk is the Internet country code top-level domain for the United Kingdom. , it is the fourth most popular top-level domain worldwide , with over 9.5 million registrations....

 namespace a college or other academic institution would register under the .ac.uk ccSLD, while companies would register under .co.uk.

See also

• Country code second-level domain
  Country code second-level domain
  A country code second-level domain is a second-level domain to a country code top-level domain. A ccSLD may be reserved by the domain name registry for the registration of third-level domains or assigned to a third party as a subdomain....
  
  
• Single-letter second-level domains
  Single-letter second-level domains
  Single-letter second-level domains are domain names in which the second-level domain consists of only one letter, such as x.com. In 1993, the Internet Assigned Numbers Authority explicitly reserved all single-letter and single-digit second-level domain names in the top-level domains com, net, and...
  
  
• Domain Name System
  Domain name system
  The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
  
  
• Top-level domain
  Top-level domain
  A top-level domain is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a...
  
  
• Country code top-level domain
  Country code top-level domain
  A country code top-level domain is an Internet top-level domain generally used or reserved for a country, a sovereign state, or a dependent territory....
  
  
• Subdomain
  Subdomain
  In the Domain Name System hierarchy, a subdomain is a domain that is part of a larger domain.- Overview :The Domain Name System has a tree structure or hierarchy, with each node on the tree being a domain name. A subdomain is a domain that is part of a larger domain, the only domain that is not...
  
  (including "third-level domain" etcetera)