Second-level Domain

A Second Level Domain (SLD) is the part of the domain name that is located right before a Top Level Domain (TLD).

For example, in mozilla.org the SLD is mozilla and the TLD is org.

A domain name is not limited to a TLD and an SLD. Additional subdomains can be created in order to provide additional information about various functions of a server or to delimit areas under the same domain. For example, www is a commonly used subdomain to indicate the domain points to a web server.

As another example, in developer.mozilla.org, the developer subdomain is used to specify that the subdomain contains the developer section of the Mozilla website.