* A name that identifies a computer or computers on the Internet. These names appear as a component of a Web site's URL, e.g. en.wikipedia.org. This type of domain name is also called a hostname.

* The product that domain name registrars provide to their customers. These names are often called registered domain names.

* Names used for other purposes in the Domain Name System (DNS), for example the special name which follows the @ sign in an email address, or the Top-level domain names like .com, or the names used by the Session Initiation Protocol (VoIP), or DomainKeys.

* They are sometimes colloquially (and incorrectly) referred to by marketers as "web addresses".
[From Wikipedia, the free encyclopedia]