A TLD is a top level domain, which usually has lots of subdomains, and, in practice, is the word following the last dot of the domain name.

A top-level domain (TLD) is the highest level of the hierarchical domain name system (DNS). They are sometimes called domain name extentions. In practice it's the word following the last dot of a domain name, e.g. .com, .org and .net.

For example, com, org and net are technically domains in their own right. However, these are TLDs.

TLDs often have lots and lots of subdomains, called Second Level Domains (SLDs). And they have subdomains as well, called Third Level Domains, but we don't have an acronym for that yet. It keeps going on from there. Here is a diagram to explain:

diagrams showing TLDs and SLDs

There is a very very long list of all the TLDs.