SLD - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

An SLD (Second Level Domain) is the domain name that is located right before a TLD.

For example, in mozilla.org, mozilla is the second-level domain of the .org TLD.

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.