I'm not sure that it's right to just focus on applications. Indeed, in some cases, it's not clear what the application is.
I think that it would be reasonable to consider some operating systems to be "enterprise software". AIX, Solaris, VMS, perhaps even Win2k and Win2k3. And some OSes quite clearly aren't "enterprise software", such as Windows 95.
Nor is it right to exclude sysadmin support software as some people have done. I don't care what your applications are, they won't deliver the goods if the sysadmins can't, for example, get early warnings about machines running out of disk space.
As for the boundary between application and OS - some people joke that emacs is their operating system, and Unix is just the device drivers. I think they have a point. If you have a machine dedicated to running Oracle, with Oracle managing the storage, Oracle managing all the memory - it seems to me that Oracle is the operating system and the applications are what you build on top of it. Users don't interact with Oracle, they interact with the patient booking system. And the Oracle DBAs rarely interact with whatever is providing Oracle's device drivers.