The Second Level Domain, or SLD, is the data directly before the top-level domain (TLD). For example, in www.example.com, “example” represents the second level domain, as the suffix "(dot)-com" represents the TLD. The SLD is generally the portion of the URL that identifies the website's domain name. Usually, the SLD is determined and defined upon registration of a domain name. More than one SLD can function on the same IP Address.
The DNS is composed of a tree-like hierarchy. Each TLD includes many second-level domains (such as "example" in "www.example.com"); each second-level domain can include a number of third-level domains ("www" in "www.example.com"), and so on.
Some countries restrict the second level under their ccTLD. These countries require that the second-level domain be one of several pre-determined second-level domains, generally indicating what type of individual or organization may register under that SLD. For these countries, you are not permitted to register domain names under the top level domain. It must be registered under one of the pre-determined second-level domains, called a ccSLD, or Country Code Second Level Domain.
An example of this would be .co.uk, where .uk is the (cc)TLD and .co is the (cc)SLD. In this circumstance, the domain name ("example" in "example.co.uk") would be considered the third level domain, as it is in the third position. This can become confusing, as the domain name is commonly equated with the second level domain, and the subdomain is commonly equated with the third level domain. In these circumstances, the third level domain is sometimes referred to as a functional SLD.