Pixels Per Inch, a measurement of display resolution indicating how many pixels are contained within one inch of screen space.
Technical acronym from 'Pixels Per Inch', coined in the 1980s with the rise of computer displays and digital imaging. The concept parallels older print industry measurements like DPI (dots per inch).
PPI is why your smartphone screen looks so much sharper than your laptop despite being smaller - it's not about total pixels but how densely they're packed. The human eye can distinguish up to about 300 PPI at normal viewing distance, which is why Apple called their 326 PPI displays 'Retina'.
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