Second-level domain
In the Domain Name System (DNS) hierarchy, a second-level domain is a domain that is directly below a top-level domain (TLD). For example, in en.wikipedia.org, wikipedia is the second-level domain of the .org TLD.
Second-level domains commonly refer to the organization that registered the domain name with a domain name registrar or ccTLD registrar. Some TLDs introduce a second-level hierarchy that indicates the type of organization registered under it. For example, in the .uk TLD a college or other academic institution would register under .ac.uk, while companies would register under .co.uk.