Middleware is a conveniently fuzzy concept, that usually means software that sits between the basic operating system and the application in a networked environment and provides some sot of service to the application.
When the term become more common in the 1980:s it most often was used to describe software that hided the complexity of the network behind a simple abstraction like making a subroutine call over the network appear like a (typically slower) version of a local subroutine call.
This site tends to interpret the term broadly, partly because things like the World Wide Web did not even exist when the term was coined and partly because we want to include things we feel is relevant even though they does not quite fit in with the classical definition.
Updated 16.4.2007 /CML