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 or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm).
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 …