A subdomain is part of a larger domain, usually separated by a dot and at the beginning of a domain name.
example.com – example is the domain name
test.example.com – test is a subdomain under example.com
As test.example.com can lead to a different website than example.com it is essentially a different URL altogether. However, the owner of example.com would be the only one able to create any subdomains (such as test.example.com) and so you can be sure that the subdomain is related to the main domain.
Most people use subdomains to separate out their site in a helpful way. For example, if a company has a strong brand name, and lots of products they might use subdomains to split them out.
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