https://en.wikipedia.org/wiki/Application_server
For Web applications, these components are usually performed in the same running environment as their web server(s), and their main job is to support the construction of dynamic pages. However, many application servers target much more than just Web page generation: they implement services like clustering, fail-over, and load-balancing, so developers can focus on implementing the business logic.
Application servers — developed before the ubiquity of web-based applications — expose back-end business logic through various protocols, sometimes including HTTP, and manage security, transaction processing, resource pooling, and messaging.[