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 isn't also a subdomain is the root domain. For example, "mail.example.com" and "calendar.example.com" are subdomains of the "example.com" domain, which in turn is a subdomain of the "com" top-level domain (TLD).

Subdomains are commonly used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as mail.cs.example.edu or www.cs.example.edu.

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