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In the Domain Name System (DNS) hierarchy, a second-level domain (SLD) 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 organization 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.
See also 
- Country code second-level domain
- Single-letter second-level domains
- List of second level domain suffixes
- Domain Name System
- Top-level domain
- Country code top-level domain
- Subdomain (including "third-level domain" etcetera)
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