An SLD (Second Level Domain) is a domain that is hierarchically directly below a TLD.
It is usully used as a way to differenciate various functions of a server or to delimit areas under the same domain. For example,
www is a commonly used SLD to indicate the domain points to a web server.
As another example, in
developer SLD is used to specify that the subdomain contains the developer section of the mozilla website.
- SLD on Wikipedia