Seeing how this thread is quickly turning into a dumpster fire I might regret adding my 2 cents, but it is filled with opinions mixed with misinformation so I thought I'd try to clear up a few things.
Let's start with the basics:
Resolution refers to pixel density, meaning the number of pixels within a given unit of distance. Since the beginning of computer-assisted graphic design in the early 1980s, the industry standard that is used all over the world is:
- 72 pixels per inch for screen design (screen or low-res)
- 150 pixels per inch for standard printing (standard res)
- 300 pixels per inch for high quality printing such as art prints and high end magazines (high resolution)
Depending on the metric or imperial system it might be pixels per centimeters, but the resolution remains the same.
Size refers to the width and height. If a printed image is 4 inches by 6 inches, you know exactly how big it is. However, the amount of pixels within the image on a screen can change drastically, which would obviously alter the quality of the image.
272×340, 312×390, 960×640 (That's the iPhone 4, is this what we call high resolution?), 1136×640, are all considered retina displays.
Those are screen sizes, not resolutions. Again, I base this statement on the accepted language of graphic design all over the world. If you ask a printing company to print something at a resolution of 960×640, they will not be able to do it; they need pixels per inch.
This is why when we create a new document we need 3 numbers: width, height and resolution.
Where does the confusion come from?
I believe it starts with the fact that what manufacturers call screen size is the diagonal measurement between 2 opposite corners, and that is an even bigger marketing gimmick than using a brand name to refer to a screen because it really tells us nothing. However, it's a measurement that everyone understands and that is easier to sell. Therefore, we say this screen is 27 inches instead of saying this screen is 5120 pixels wide by 2880 pixels high at a resolution of 190 pixels per inch. However, as graphic designers, size and resolution is our science; it's the precise data that we use to be efficient.
Let's look at the table you have linked:
If we take the iPhone 4 like the example in the question, the screen size is 3.5 inches, or 89mm. Again, this tells us nothing as it could be 0.5 inches wide by 3 inches tall, or it could be a square screen about 2.5 inches wide.
What Wikipedia calls resolution here should be the screen size: at 960x640 with a resolution of 326 ppi, you know that the screen is exactly 2.95in by 1.96in. Therefore, if you create an image at that size with a resolution of 326 ppi it will fill the screen and it will look exactly the same as you have designed it. If all you have is 3.5 in, as a graphic designer there is nothing you can do with that information.
Therefore, based on the standards of graphic design that have been used for decades, Cai is right in saying that the iPhone 4 is high resolution as it has a resolution of 326 pixels per inch.
Besides it's redundant and confusing. EX: the new iMac is called "iMac with Retina 5K Display" So what is it? a 5K display. What does retina mean in that context? Nothing at all.
In this context, retina 5K refers to the fact that the 27 inch screen is 5120 by 2880, as opposed to the retina display on the previous 27 inch iMac which was 2,560 by 1,440.
So what does this all mean for a graphic designer?
Let's take the example in the question:
It encourages users to creates poor questions like this one:
"Retina scaling of icons" <- link dead, but title relevant
Before all the fancy new displays, we could get away with creating a favicon that had only 3 sizes:
- 16x16: browser favicon
- 32x32: taskbar shortcut icon
- 96x96: desktop shortcut icon (and Google TV)
Today, we have also to consider:
- 120x120: iPhone Retina (iOS 7)
- 180x180: iPhone 6 Plus (iOS 8+)
- 152x152: iPad Retina (iOS 7)
- 167x167: iPad Pro (iOS 8+)
And a few more examples for good measure:
- 128x128: Chrome Webstore icon
- 196x196: Android Chrome icon
- 228x228: Opera Coast icon
And those are all at the standard screen resolution, which has always been 72 dpi. As we can see in the table above, every Retina screen is different so the question is not that poor, there is a lot to say on the subject. The question has been deleted, but asking "What size and resolution should I use to create icons for apple devices" does not seem poor at all and could be very informative.
Is the tag relevant?
It's a marketing gimmick not a unit of measurement
It's a made up trademark by Apple.
If you consider every registered brand name to be a marketing gimmick then yes, it is. As shown above, it clearly is not a unit of measurement. It is indeed a trademarked name by Apple for their screens which are different than others.
What we call Retina is basically a new generation of screens. Even within the Retina brand name there has been a few different iterations: Retina Display, Retina HD Display, Super Retina HD Display, or Retina 4K/5K Display.
It's arbitrary within Apples own usage though. What apple considers high res today is not high res tomorrow. Meaning the answer and questions themselves quickly become obsolete and irrelevant. Even within it's own eco-system it's broken.
A new industry standard for what is considered high resolution and a new standard name for the family of displays that are currently known as Retina are 2 completely different things.
In the world of graphic design, what we call high resolution has always been 300 dpi. With all the new screens coming out this might change, but so far it has been the standard graphic design language since the early 1980s and it is based on the printing industry.
Maybe Retina will become the new industry standard and there will be a new name for it, just like we don't call every smart phone an iPhone and we now say tissues instead of Kleenex. The Retina tag could be renamed to something like "New generation of screens with unusual resolution and pixel sizes", but at the moment they are in a separate class, specific to Apple, with precise technological criterias that cannot be ignored by graphic designers. You might consider Apple a special snowflake for giving a fancy name to their fancy screens, but they have such a big market share that chances are your client will see your design on a Retina screen unless you design for print only, and those screens have specific characteristics.
Just like the Webmaster stackexchange site has an Internet Explorer tag; it could be merged with Web browsers, but it has its own particularities.
The purpose of tags is to specify what the question is about. Every tag could technically be merged with another, more general tag until we end up with a single tag called Graphic design. The Retina tag is certainly not the best, but merging it with Resolution would be less precise than keeping it, especially since a question can have more than one tag. A question could be about general resolution, about Retina screens only (for example about color calibration) or both and be about the resolution on retina screens.