We are working on an offline application for which we use a web interface as the GUI. We have sensitive data showing there, so it is important for us that in our communication we make clear that this is a desktop application, "something offline, not in the internet available to everyone". The web interface is just because it was most useful for the GUI service we wanted to build. Our audience is also very heterogeneous, with a lot of non-technical people.
I am not an English native speaker nor a trained web developer, so I find it difficult to get the correct term, which is not too complicated to understand:
"Website" is surely incorrect - "web service" as well.
"Web application" could work or "offline web application" (latter has a taste of "broken tool" to me, though).
What do you think?
