PPI (Pixels per inch) (I have got the information from the wikipedia then I made it in my own words)

Pixels per inch or in a short term called PPI or pixels per centimeter

that are called PPCM in short is the size of the pixel resolution of an

automatic image device, likes a computer screen or television screen,

or image digitizing device such as a camera or image scanner.

Horizontal and vertical density are usually equivalent to each other

as most devices have square pixels, but on some devices it could

be different that doesn`t have non-square pixels. PPI can as well define

the resolution, in pixels, of an image file.