Dots per Inch (DPI) is the standard measurement used for printers. It refers to the amount of dots that are placed by the printer in a one inch space. This is similar to Pixels Per Inch (PPI), which is used to measure how many pixels are placed in an inch by a monitor or screen. Aspect/Contrast Ratios are used to compare height and width of paper, monitors, etc. For example 4x1 would be 4 height for every 1 width - this could be 400x100 or 800x200.
Dots per inch (DPI, or dpi) is a measure of spatial printing, video or image scanner dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (254 cm) Similarly, the more newly introduced dots per centimeter (d/cm or dpcm) refers to the number of individual dots that can be placed within a line of 1 centimeter (≈ 0393 in) Monitors do not have dots, but do have pixels the closely related concept for monitors and images is pixels per inch or PPI Many resources, including the Android developer guide, use the terms DPI and PPI interchangeably