An application server is a server that hosts applications[1] or software that delivers a business application through a communication protocol.[2] For a typical web application, the application server sits behind the web servers.
An application server framework is a service layer model. It includes software components available to a software developer through an application programming interface. An application server may have features such as clustering, fail-over, and load-balancing. The goal is for developers to focus on the business logic.[3]
an application server provides an environment where applications can run, no matter what the applications are or what they do
[A]n application server exposes business logic to client applications through various protocols