Watermark 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.

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

Watermark n. : See Water line, 2.

Example:
How do I create a Watermark using Gimp or any other free software?

Taxonomy
watermark, paper, Dandy Roll, wires, stamp, Mould, Watermark Paper, free encyclopedia, thickness, digital watermark, The Cylinder, Audio watermark, watermark detection, physical paper, postage stamps, chain, appearance, thinner.

Defination from the web:
watermark is a recognizable image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or ... There are two main types of watermark, the Dandy Roll process, and the more complex Cylinder Mould process. ...