I continue working on the Term Paper.
The more I read about ESB, the more I can see how controversial the topic is.
People cannot agree on almost everything related to ESB.
For example, some people think that an ESB is not a product, but an architectural pattern.
Others, however, describe ESB as a coherent piece of infrastructure available for purchase through a number of vendors.
I believe that both notions are correct. ESB is definitely an architectural pattern with some distinctive features, but at the same time, it is a coherent piece of software and not just a composition of traditional middleware in a new packaging.
I was also finally able to create a short homemade ESB definition based on the Dave Chappel's article: “ESB is a SOA-based Enterprise Application Integration Broker”. I think that the definition above gives a good idea of what ESB really does and what role it plays in the enterprise architecture.