Watermark

n. mark indicating the height to which water has risen; design that is impressed in paper and is only visible when held up to the light

v. make a watermark (on paper); impress a design in paper which is only visible when held up to the light

A watermark is a recognizable image or pattern in paper that appears lighter when viewed by transmitted light (or darker when viewed by reflected light, atop a dark background). A watermark is made by impressing a water-coated metal stamp or dandy roll onto the paper during manufacturing. Watermarks were first introduced in Bologna, Italy in 1282; they have been used by papermakers to identify their product, and also on postage stamps, currency, and other government documents to discourage counterfeiting.
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Noun
1. a line marking the level reached by a body of water
(synonym) water line
(hypernym) line
(hyponym) high-water mark
2. a distinguishing mark impressed on paper during manufacture; visible when paper is held up to the light
(hypernym) marker, marking, mark

(n.)
See Water line, 2.
  

(n.)
A mark indicating the height to which water has risen, or at which it has stood; the usual limit of high or low water.
  

(n.)
A letter, device, or the like, wrought into paper during the process of manufacture.
  

Synonyms and related words:
Plimsoll line, Plimsoll mark, birthmark, blaze, blemish, blotch, brand, caste mark, check, checkmark, cicatrix, cut, dapple, discoloration, dot, earmark, engraving, fleck, flick, freckle, gash, graving, hack, high-water mark, jot, lentigo, load waterline, macula, mark, marking, mole, mottle, nevus, nick, notch, patch, point, polka dot, prick, puncture, scar, scarification, score, scotch, scratch, scratching, speck, speckle, splash, splotch, spot, stain, stigma, strawberry mark, tattoo, tattoo mark, tick, tidemark, tittle, waterline