Apple actually made a worthwhile distinction with "retina display" - it's the resolution at which the human eye can't discern different pixels from the distance

Apple actually made a worthwhile distinction with "retina display" - it's the resolution at which the human eye can't discern different pixels from the distances it will be used at. Simply noting the DPI doesn't capture this. Some argue that since the iPad is often held further away, it could have lower DPI than the iPhone and still have indiscernible pixels.