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 centimetre (d/cm or dpcm) refers to the number of individual dots that can be placed within a line of 1 centimetre (≈ 0393 in)