Definition:
A subdomain is a more specific portion of a domain name. Subdomains are used to divide up Web domains without registering a new domain name.
Subdomains use the Web server to define the location of files for that site, rather than using DNS. The easiest way to do this is to set up virtual hosting on an Apache Web server.
Alternate Spellings: sub-domain
Examples:
Subdomains are the most specific portion of the domain name, so they are found in the left-most portion of the domain name. Such as:
webdesign.about.com
www.about.com