What is a third-level domain?
A third-level domain is the section of a domain name that is to the left of the dot of a second-level domain (SLD) that is considered a domain extension. For example, .CO.UK is a country code second-level domain (ccSLD) under the .UK country code top-level domain (ccTLD). The .UK registry supports domain registration under .CO.UK with ".CO.UK" serving as the domain extension. For example, we own Dynadot.CO.UK, so our third-level domain is "dynadot".
Please note that a third-level domain is different than a subdomain.