Subdomain

Definition by Web Hosting Ratings:

Subdomain is a way to divide your site into sections with short and easy to remember names.  For example, a section of this site for new users could be at newbies.webhostingratings.com.  Other use of subdomains might be to let somebody else use your account (but this may not be allowed by your host's terms of use).  Large websites might make their subdomains point to another server to reduce load on the main www site.