The Domain Name System dns has a tree structure or hierarchy, with each node on the tree being a domain name. A subdomain is a domain that is part of a larger domain, the only domain that is not also a subdomain is the root domain.
calendar.example.com are subdomains of the
example.com domain, which in turn is a subdomain of the
com top-level domain (TLD).
A "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org.