dpi is an abbreviation for dots-per-inch. It is used as a measure of the capability of printers - a 1200dpi laser printer is markedly better than a 300dpi laser printer.
Do not confuse dpi with screen ruling. An offset litho press will print with a screen ruling of, say, 120 lines per inch - or dots per inch. To make a dot that is large enough to be useful on litho printing plate quite a number of laser printer dots are required - maybe 25 or more.
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