What does "watermark" mean?

Watermark: A letter, device, or the like, wrought into paper during the process of manufacture. Note: "The watermark in paper is produced by bending the wires of the mold, or by wires bent into the shape of the required letter or device, and sewed to the surface of the mold; -- it has the effect of making the paper thinner in places. The old makers employed watermarks of an eccentric kind. Those of Caxton and other early printers were an oxhead and star, a collared dog's head, a crown, a shield, a jug, etc. A fool's cap and bells, employed as a watermark, gave the name to foolscap paper; a postman's horn, such as was formerly in use, gave the name to post paper." Tomlinson.