From a DNS perspective, a domain is simply way to address any and all systems that have been grouped together under one "dot com". For example, www.hotpr0n.com and mail.hotpr0n.com are both hosts within the hotpr0n.com domain.

Confusingly this is also the term that Sun Microsystems use to refer to a group of components that have been grouped together to form a single logical system within an Enterprise 10000 computer.