This brings up another problem. The term printing can mean two things. The first is the process of duplicating books, magazines, newspapers, etc., on a printing press. The problem can arise because in the old halftone process a dot was in a sense a pixel, so the term pixels per inch and dots per inch could be used interchangably. This is NOT the case for an ink-jet or laser printer.
Normal images take a number of dots to create each pixel.
The halftone process. After the invention of the printing press the printing of images required a laborious preparation of a plate. One cannot directly print lots of shades of grey using black ink on a printing press. An image plate was created by hand carving a flat-surfaced block of wood, or engraving a metal plate. Dark areas were depicted by closely spaced fine lines.