A Retina display is more of a proprietary, Apple-specific marketing term than a precise technical term, but there is a definition: it refers to a screen on a computing device that has a high enough pixel density that the human eye can't make out individual pixels - or a general 'pixellation' effect - at all. In other words, the human eye is scientifically incapable of telling the difference between a photo of a painting shown on a Retina display, and the painting itself - in theory, anyway.
My eye won't be fooled.
That varies. Screen resolutions are given in the format '[number of pixels] x [number pf pixels', but the key factor in classifying a screen as Retina is pixel density, not the overall number of pixels. This makes sense, if you think about it: if you spread the same number of pixels across a larger screen, it will obviously be easier for the eye to pick out individual pixels. Pixel density is given as a single figure, measured in pixels per inch, or ppi.
Yes there are. At any rate, there are screens out there offering a higher pixel density in the various categories listed above. Most obviously, since the iPhone 6 launch, Apple itself offers an upgraded class of screens that it calls 'Retina HD'. But many Android devices surpass not only the Retina screens but even the Retina HD models. The LG G3, for instance, has a flabbergasting pixel density of 534ppi, which blows even the iPhone 6 Plus out of the water.
The term Retina HD display - so far - refers only to the screens on the iPhone 6 and iPhone 6 Plus, although it seems almost certain that the iPhone 6s or iPhone 7 will boast similar specs - and perhaps 2015 will be the year that Apple extends the Retina HD branding to the iPad line. The iPad Pro, for one, would benefit from the cachet and differentiation that Retina HD would offer, but the iPad Air 3 and iPad mini 3 are also possibilities.
So what is the difference between Retina and Retina HD? Apple has been a little vague on this, but it appears to refer to resolution rather than pixel density, meaning that bigger (but not necessarily sharper) screens qualify.
Obviously this depends on the resolution, pixel density and so on of the non-Retina display, but it's a fairly safe bet that Apple will never sell a computing device with a fuzzy or unclear screen.
Here are the product areas where Apple offers Retina displays, alongside any non-Retina alternative(s):
iPad mini 2: Retina display. From £239 with 16GB.
As with the iPads, there are no non-Retina options here: all iPhones currently available have at least Retina displays. The concept was introduced with the iPhone 4, and was also present on the iPhone 4s and iPhone 5.
12in MacBook: Retina (from £1,049).
13in MacBook Pro: non-Retina (from £899).
All of Apple's MacBook Air laptops are non-Retina. But there is a persistent rumour that a Retina MacBook Air will launch in the near future.
Let's start with plain old Retina.
What about Retina HD? That's harder to quantify, since the iPhone 6 and iPhone 6 Plus are such different propositions. They're lovely screens, for sure, but the main thing they offer over their non-Retina predecessors is size. The iPhone 6 Plus is super, super sharp, but the iPhone 6 has the same pixel density as the iPhone 5s.