What is a domain?

That still does not explain what a domain is.

In JBoss AS, you can deploy an application by simply copying it to the server/xxx/deploy directory. For example, if you deploy foo.war, you can access it using the url http://localhost:8080/foo. Not sure if that answers your question.

If a domain in weblogic is the same as a domain in glassfish and the same as a component in the OracleAS then I think the poster is asking whether you can create a completely separate J2EE container within the same installation of JBoss which allows you to deploy the same EAR file into a domain/component for development purposes then into another for testing and possibly a third for production.

Another use of domains is that you can implement application hosting much more easily i.e. you have a domain A for Customer A a domain B for customer B completely separate from each other running the same application in the same instance of your application server. You define a datasource A for Customer A in domain A and a datasource B for customer B in domain B.

Glassfish provides a very nice admin console for this (as does WebLogic).

I have also seen the term "domain" used to refer to what is called a "configuration" in JBoss AS. NetBeans is an example - when you select the "domain" for the app server it lists the configurations in the server directory. So yes, you can copy "default" as "domain1" and then "run -c domain1" provided you have changed the configuration to prevent port conflicts.

But perhaps the "domain" mentioned here is closer to the idea of virtual hosts, which is described here: http://www.jboss.org/community/docs/DOC-12549