A space—or keyline—left in a document for the later insertion of some form of graphic image. Also known as a cutout.
The term knockout is also used to refer to "white type" or, in other words, type which prints as a reverse, or, in fact, doesn't really print at all, allowing the color of the page to show through a background in the shape of type. Sometimes pelled as two words, knock out, or with a hyphen, knock-out.