Domain

Internet-speak for a group of computers. Normally, all computers in a domain are able to communicate with one another (they may be on the same network, or on networks that are interconnected). Domains also share a common login authentication system.

Domains have names, called Domain Names. Within a domain, individual computers (hosts) must have names too, called Host Names.  Within the Internet, host names are combined with the name of the domain to which a computer belongs, forming the Fully Qualified Domain Name (FQDN). For example, a host named "www", which is a member of the domain "microsoft.com" would have a FQDN of "www.microsoft.com".