Topic summary
Pixels per inch
Extracted from the Wikipedia article Pixel density.
Basic principles
Pixels per inch (or pixels per centimetre) describes the detail of an image file when the print size is known. For example, a 100 × 100 pixel image printed in a 2-inch square has a resolution of 50 pixels per inch. Used this way, the measurement is meaningful when printing an image. In many applications, such as Adobe Photoshop, the program is designed so that one creates new images by specifying the graphic output device and PPI (pixels per inch). Thus the output target is often defined upon creating the image.