In the Domain Name Server (DNS) hierarchy, a subdomain is a domain that is part of a larger domain name. A DNS hierarchy consists of the root-level domain at the top, underneath which are the top-level domains, followed by second-level domains and finally subdomains.
Valid subdomains as described by RFC 1034 may contain any letter or number as well as the '-' (hyphen), but must begin with an alphabet character. While both upper and lower case characters are valid, subdomains are case insensitive so 'ABC.com' is identical to 'abc.COM'.
The length of a particular subdomain label is restricted to 63 characters, but the number of subdomain labels in a domain name is arbitrary.
Structure
Subdomains are separated by dots "." and are read from left to right, in descending specificity. Each consecutive subdomain contains all previous subdomains, and is contained by any subsequent subdomains. All subdomains are contained by the second-level domain, which is in turn contained by its top level domain.
Example:
subCBA.subBA.subA.domain.tld
In this example the top level domain is 'tld' and the second level domain is 'domain'. Within 'domain' the subdomains are contained in a tree structure.
Popular real-life examples would be news.google.com, mail.yahoo.com, support.exabytes.com and en.wikipedia.org, in which news, mail and en are subdomains, respectively.