What is the difference between domain, subdomain and third level domain?

What is a domain?

The domain (e.g., example.com) is the overarching structure that contains the subdomains (e.g., blog.example.com). For example, example.com, google.com and joturl.com are domains.

What is a subdomain?

A subdomain is a domain that is part of a larger domain, for example, west.example.com, go.link.example.com and east.example.com are subdomains of the example.com domain.

What is a third level domain?

A third level domain is a specific subdomain that has only 3 parts. For example, west.example.com, go.example.com and east.example.com are third level domains (and even subdomains) of the example.com domain, but go.link.example.com is a subdomain but it is not a third level domain as it has 4 parts (actually it is a fourth level domain).