For some top-level domains such as .com, .net and .org, second-level domains (2LDs) can be registered. When registering one of these, you define the portion left of the dot. example.com
If you consider com.au a top level domain name, example.com.au would be a second-level domain name however example.com.au is usually considered a third-level domain name (3LD) while com.au is the 2LD and .au is the top-level domain name.