Retina
Apple's marketing name for displays with pixel density high enough that individual pixels can't be perceived at normal viewing distance. Launched with iPhone 4, 2010.
Steve Jobs's definition: ~300 PPI at 10–12 inch distance. Not a standard, just Apple's brand. Similar tech: Sony Triluminos, Samsung AMOLED Plus. Retina led the industry toward HiDPI ⇒ Windows/Linux had to catch up on OS scaling support. Apple Retina XDR = mini-LED/OLED variant with HDR. devicePixelRatio in CSS/JS is how web developers handle Retina.