watermark -
Auxiliary data embedded in a datafile for subsequent identification and verification of the file.
Verb
watermark -
To mark paper with a watermark.
watermark -
To mark a datafile with a watermark.
Wikipedia
A watermark is a recognizable image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper. There are two main ways of producing watermarks in paper; the dandy roll process, and the more complex cylinder mould process. Watermarks vary greatly in their visibility; while some are obvious on casual inspection, others require some study to pick out.
OmegaWiki Dictionary
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watermark A translucent design impressed on the surface of paper during manufacture and visible when the paper is held to the light.