WSGI

http://en.wikipedia.org/wiki/Web_Server_Gateway_Interface

The WSGI has two sides: the "server" or "gateway" side, and the "application" or "framework" side. To process a WSGI request, the server side provides environment information and a callback function to the application side. The application processes the request, and returns the response to the server side using the callback function it was provided.

So-called WSGI middleware implements both sides of the API so that it can intermediate between a WSGI server and a WSGI application: the middleware acts as an application from some WSGI server's point of view and as a server from some WSGI application's point of view.