Learn in the textbookFundamentals of Color and Image Representation
Definitiondpi (dots per inch) is a unit of resolution expressed as the number of dots lined up across one inch (about 2.54 cm).
ExplanationThe larger the value, the more finely the dots are packed, allowing more detailed scanning and printing. On an image scanner each dot read becomes one pixel of the image, so each side yields "length (inches) × dpi" pixels. Since the pixel count is width times height, doubling the dpi makes it four times as large.
AnalogyIt is like graph paper of the same size: the finer the grid, the smoother the picture you can draw on it.
Memory hookOne inch is roughly the width of an adult's thumb.