Retina does not take viewing distance into account. There is no definition of the term retina as far as I know. Apple just uses it for all their displays with higher pixel densities.
The definition is pretty clear: The pixel grid is not visible.
For that three variables have to be taken into account: screen size, screen resulution and viewing distance. Actually, four: visual acuity, too (though it makes sense to just set that at 20/20).
That's the definition Apple repeadtedly gave in so many words. They were actually pretty explicit about that. Sure, it's their trademark. They are in no way obligated to respect that definition. They can turn around tomorrow and call every one of their screen a retina screen - but when people use the term retina they usually mean Apple's original definition.
It's a squishy term, sure, but I like it. It makes a ton of sense since it puts human perception on the center stage and that's what matters. Human perception is squishy, so it makes sense that the retina term would also be squishy.
"The pixel grid is not visible."
human perception is different for individual humans. Is a 10 inch 1024x768 display already retina, because many humans cannot see the pixel grid? Or is the 3.5 inch iphone 4 not retina, because a very healthy human eye can still see the pixels?
The distance is also very squishy attribute. From my experience, people look at a 10 inch tablet from about the same distance than they look at a smartphone. Still, Apple gave the ipad a lower minimum requirement for the Retina label than the iphone 4.
And does this mean I can "retina" and "unretina" a screen by moving closer/away from the screen?
Ah! The STEM person and their inability to deal with squishiness! Lovely, just lovely!
You have to ask yourself one thing: What actually matters when it comes to resolution? Human perception does, of course! Everything else is pointless. Human perception is inherently squishy, that's just how it is. There really is no way around that.
If you start talking resolutions without a more complete understanding of human perception giving you context your talk is just meaningless.
The retina term here provides an easy summary, nothing more. There are always edge cases, sure, but the retina term is both explicit and unspecific enough to work very well. That's why I like it so much.
Yes, distances are different - but not so different. Yes, visual acuity is different and that sucks - but the simple solution here is to just pick 20/20. Maybe a bit better.
There are no simple yes/no answers here - but I don't think there have to be.
