Definition - What does Third-Level Domain mean?
A third-level domain is the next highest level following the second-level domain in domain name hierarchy. It is the segment that is found directly to the left of the second-level domain. The third-level domain is often called a "subdomain", and includes a third domain section to the URL.
In large organizations, every department or division may include a unique third-level domain that can act as a simple, yet effective, way of identifying that particular department.
Various third-level domain names are used to balance the load on sites with heavy traffic. At times, names like www1 or www2 are used for this objective.
Techopedia explains Third-Level Domain
For example, in www.mydomain.com, "www" is the third-level domain. The default or the most commonly used third-level domain is "www". The third-level domain is generally used to mention a certain server inside a company.
Domain names are made with a minimum of two levels, a top-level domain (TLD) and a second-level domain. TLD is the extension or suffix attached to the domain names. Only a small selection of predefined TLDs are available, including .com, .org, .net, .biz, etc.
A second-level domain refers to the part of a Uniform Resource Locator (URL) that specifies the precise administrative owner linked to an Internet Protocol (IP) address. The second-level domain name incorporates the TLD name as well. For example, in www.mydomain.com, ".com" is the TLD and "mydomain.com" is the second level domain.
Third-level domain names are not mandatory unless the user has a specific requirement. It is actually possible to own a fully functional domain name like "mydomain.com". All that is needed is two levels: the top level domain and the second-level domain name. However, the usage of third-level domain names can really add clarity to domain names, which makes them more intuitive.
Customized third-level domains are also used for specific purposes. For instance, if mydomain.com has a file transfer protocol (FTP) server to let users download files, its third-level domain name can be termed ftp and the full domain name would then be ftp.mydomain.com. Similarly, the domain names such as support.mydomain.com and members.mydomain.com can be employed to differentiate the support department and member department of mydomain.com respectively. This will help direct the Web traffic accordingly.