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.