A second-level domain name is the first level at which a domain name may be registered. The TLD (.com, .net, .org, etc) is the first level of a domain name but these cannot be registered to anyone.
Located immediately to the left of the dot (” . “), the second-level domain is the “readable” part of the domain name. The second-level domain is entirely defined by the registrant.
For example: In cnn.com, “cnn” is a second-level domain name.
Second-level domains can be divided into further domain levels.
For example: sportsillustrated.cnn.com. These third-level domains sometimes represent different computer servers within departments.