Dots per inch

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, will use the term DPI interchangeably with PPI, however.

DPI measurement in monitor resolution in pixels

Old CRT type video displays were almost universally rated in dot pitch, which refers to the spacing between the sub-pixel red, green and blue dots which made up the pixels themselves. Monitor manufacturers used the term "dot trio pitch", the measurement of the distance between the centers of adjacent groups of three dots/rectangles/squares on the CRT screen. Monitors commonly used dot pitches of 0.39, 0.33, 0.32, 0.29, 0.27, 0.25, or 0.22 millimetres [mm] (0.0087 in).

LCD monitors have a trio of sub pixels, which are more easily measured.