In the context of the OWIN Framework middleware is a class that implements IMiddleware<T>.

When a class implementsIMiddleware<T> it is making a promise to store an implementation of interface T into the OWIN context during request processing. The OWIN Framework provides an extension method that adds a SetFeature<T>() method to IOwinContext. Middleware classes should use this extension method to store T in the OWIN context during rrequest processing. This is referred to as downstream communication.

See IMiddleware for more details about the IMiddleware<T> interface.