A Microsoft file hosting and synchronization service.
IF only it were that simple. I'm signed in. That's not the issue.
The tell-tale sign that this is more than that is that I still can "View online" from within the OneDrive pop-up interface by clicking on the three dots to the right.
But this only shows files which have been recently used. So if I want to open a file that hasn't been used recently online, this won't work.