What is a subdomain?
A subdomain is a subset of a domain name. A domain name typically consists of a top-level domain (TLD) also known as a domain extension (such as .COM) and a second-level domain (SLD). For example, our domain name is dynadot.com with "dynadot" as our SLD and .COM as our TLD. An example of a subdomain would be example.dynadot.com with "example" being the subdomain.
Please note that a subdomain is not the same as a third-level domain.