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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" .

Third-level domains are commonly used for structuring and organizing different components of a website, for example in a URL such as blog.website.com, "website" is a second-level name and "blog" is a third-level domain, and it can be used for hosting a blog section on the "website.com" domain.

Please note that a third-level domain is different than a subdomain.