Definitions for "Third Level Domain"
The third level domain is the next lower level of the hierarchy underneath the second level domains. In a domain name, this is the name that appears immediately to the left of the second-most right "." In the case of the domain name "www.i-DNS.net", the third level domain is "www".
In the Domain Name System, this is the next highest level in the hierarchy underneath the second level domains. It is that portion of the domain name that appears two segments to the left of the top-level domain. Third level domains are not the portion of an email address that appears in front of the @ symbol - for example, the webmaster in [email protected] is not a third level domain.
The next highest level of the hierarchy underneath the second level domains. In a domain name, that portion of the domain name that appears two segments to the left of the top-level domain. For example, the whatever in whatever.ta.us.
Keywords:  subdomain, based, name
a subdomain based on our name
Keywords:  yahoo, alphabetic, dot, string, com
The alphabetic string before the dot and the 2LD. Between 2 and 63 characters long, this is the 'yahoo' in yahoo.com.au.