A second-level domain (SLD) is the portion of a Uniform Resource Locator (URL) that identifies the specific and unique administrative owner associated with an Internet Protocol address (IP address). The second-level domain name includes the top-level domain (top-level domain) name.

For example, in:
whatis.com

“whatis” is a second-level domain. “whatis.com” is a second-level domain name (and includes the top-level domain name of “com”). Second-level domains can be divided into further domain levels. These subdomains sometimes represent different computer servers within different departments.