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
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 subdomain is used to specify that the subdomain contains the developer section of the Mozilla website.