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The '''Second Level Domain''', or '''SLD''', is the data directly before the top-level domain ([[TLD]]).<ref>[http://www.giantgeek.com/blog/?p=97 What is a SLD]</ref> 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.<ref>[http://www.build-your-website.co.uk/choosing-domain-name-second-level.htm SLD]</ref> Usually, the SLD is determined and defined upon registration of a domain name. More than one SLD can function on the same [[Internet Protocol|IP]] Address.<ref>[http://help.godaddy.com/article/335 SLD Definition]</ref>

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.