I think Bokeh is a useful term, and like many other things, now seems to be a standard part of photographic vocabulary. But it applies to something quite subjective and not a fixed rule. Background
blur can be either pleasant or distracting. For quite a few generations, both filmmakers and still photographer (esp back in Pictorialist days) would go to considerable expense or effort to acquire lenses with a particular out-of-focus quality. Some these might be classified as soft-focus, some variable, and some otherwise sharp. More aperture blades helps the roundness of out-of-focus highlights, but there are numerous other factors. And some people still will pay a premium for qualities
they consider tasteful in this respect. I'm not a movie addict by any means, but there are times when
I will watch one more for the quality of lighting, color, and character of the lenses than for the storyline.
So much of that expertise is being lost in all these digitized action flicks! But given the big budgets in
days of yore, some of these guys really knew their lenses and how to keep attention on the intended
subject without background distraction. But trying to actually quanitfy the term will just open a can