SecondLevelDomain

Second-level domain is a term used in the Domain Name System to describe the part of a domain name that sits immediately to the left of the top-level domain (TLD). In a domain such as example.com, the second-level domain is "example" and the TLD is "com." The combination of the second-level domain and the TLD forms the core human-readable address, and any further labels to its left (such as www or mail) are subdomains that extend the full domain name, or the fully qualified domain name (FQDN).

Second-level domains are registered under a specific TLD and are managed by registries in coordination with registrars. Registration policies vary by TLD and can specify eligibility, dispute resolution processes, renewal terms, and pricing. Availability is determined on a per-SLD basis within the chosen TLD. A registered SLD often serves as a brand, organization name, or individual identity on the internet.

The concept distinguishes the second level from subdomains, which are created within an existing domain to organize services or sections of a site (for example, www.example.com or blog.example.com). In some country-code TLDs, the boundary between second-level and third-level domains differs due to local registration structures; for instance, in domains like example.co.uk, "co.uk" functions as a secondary layer under the UK TLD, and "example" may be considered a third-level label in that namespace. Internationalized domain names (IDNs) allow SLDs to contain non-ASCII characters, typically via a Punycode representation.

Overall, the second-level domain is a central element of a domain name, representing the primary identifier registered under a particular TLD.