In n-tier architecture an application server is a server that hosts an API to expose Business Logic and Business Processes for use by other applications [1]. The term can refer to:
1. The Services that are made available by the server
2. The Computer Hardware on which the Services are deployed
3. The Software framework used to host the Services such as JBoss application server or Oracle Application Server
Reference:
http://en.wikipedia.org/wiki/Application_server
