When used as a verb, quad means, in typography and typesetting, to space out the blank portion of a line to its full measure. See Quadding.
When used as a noun, quad means, in metal typography, a blank piece of metal—more formally called a quadrat—used to fill up a line of type, forcing the rest of the characters into a desired position on the line. Quads were used to set a line flush left, flush right, or centered.
In binding and finishing, the term quad is also a shorthand term for a quadruple folder. See Folding: Knife Folder.