Subdomain

From Site5Wiki

A subdomain is often confused as being a secondary domain name hosted within an account, when in fact, it is not actually a true domain name. Examples of subdomains include: www.domain.tld, ftp.domain.tld, etc.

Top Level Domain ( or TLD ): .com ( .net, .org, .biz, ...)
Domain Name or Second Level Domain:  domain.tld
Sub Domain or Third Level Domain: sub.domain.tld