It is generally know as PPI that is pixel per inch. It is used for measurement of resolution of device screen.More the PPI sharper the image.In order to make the image the size of pixels are reduced in order to increase its density.
Pixel density is a measure of how close together the pixels are packed on your display. The pixel density is related to size of the display in inches and the total number of pixels in horizantal and vertical directions.
Pixels density is a measurement of an electronic image device, such as a computer monitor or television display, or image digitizing device such as a camera or image scanner, horizontal and vertical density are usually the same, as most devices have square pixels, but differ on devices that have non-square pixels.