PPI stands for pixels per inch. It measures pixel density, which means how many pixels fit into one inch of a screen or of a printed image.
For screens, a higher PPI gives a sharper display. For prints, PPI tells you how large an image file will print. For example, a photo 3000 pixels wide printed at 300 PPI will be 10 inches wide. PPI is often confused with DPI, which counts the ink dots a printer places in one inch.
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