Fuzzy indeed. I'm not sure I understand what you mean at all. "Desktop" tends to have a fairly consistent definition in the context of user environments: It's the graphical component of the interactive facilities of your computer/operating system combination, in the state of the user having logged in (if appropriate) and being in the process of using the machine. What does that have to do with the underlying operating system supporting multiple users with independent desktops? 

"Terminals" in the Plan 9 world are just that: terminals. They are the physical computers users use to interact with the rest of the system (to a first order approximation, Plan 9 can be thought of as being something like a "timesharing system built from a network of computers"). Of note, they tend to be single-user (modulo a few processes that may run as e.g. "none" or whatever). They tend to present the user with a GUI that I would argue is a "desktop": rio, acme, etc give one access to one's files and present an interface for accessing the underlying system. While they tend not to use the bitmapped graphical icons of other systems, I argue that limiting the definition of desktops to being characterized by icons representing objects such as files and applications while being present on the screen seems like an implementation detail and unnecessarily limiting. http://pub.gajendra.net/2016/05/plan9part1

        - Dan C.