TLD stands for “Top Level Domain”. TLDs consist of letters after a dot, .com and .net. are the most common examples. There are many TLDs which are specifically for countries. These are known as “country code Top Level Domain” (ccTLDs).
Click here to see the long list of Top Level and Country Coded Top Level Domains.
The .au domain is a “top level domain” (TLD) for Australia which can also be expressed as Australia’s “country code Top Level Domain” (ccTLD).