The following is from the PC Webopaedia (http://www.pcwebopaedia.com/TERM/m/middleware.html):

Software that connects two otherwise separate applications. For example, there are a number of middleware products that link a database system to a Web server. This allows users to request data from the database using forms displayed on a Web browser, and it enables the Web server to return dynamic Web pages based on the user's requests and profile.

The term middleware is used to describe separate products that serve as the glue between two applications. It is, therefore, distinct from import and export features that may be built into one of the applications. Middleware is sometimes called plumbing because it connects two sides of an application and passes data between them. In a three-tier architecture, middleware occupies the middle tier. Common middleware categories include:

TP monitors DCE environments RPC systems Object Request Brokers (ORBs) Database access systems Message Passing