Pixel per inch (ppi) is a quality specification for the resolution capacity of displays, televisions, digital cameras, scanners and digital images. Pixels per inch (ppi) refers to the pixel density, which is reflected in the digitized pixels per inch.
The higher the pixels per inch, the higher the image resolution; the lower it is, the coarser the scanned or displayed digital image. An image with coarse resolution will show pixel stairs, but these will not be visible at a greater viewing distance.
Image resolution depends on the device or image display. To avoid seeing pixel steps in small photo prints, digital photos should be printed at a minimum of 300 ppi. For posters viewed from a greater distance, 150 ppi is sufficient, and even 75 ppi for displays. As for the resolving power of displays, these are around 130 ppi for screen sizes of 30 inches, and 400 ppi for high-resolution smartphones like the iPhone 6. In addition to pixels per inch (ppi), there is also the term dots per inch( dpi), which is used synonymously.
In German literature, pixel or dot density is also specified in metric units as pixels per centimeter (ppcm) or pixels per millimeter (ppmm).