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 very useful in the examination of paper because it can be used for dating, identifying sizes, mill trademarks and locations, and the quality of a paper.
Watermarks vary greatly in their visibility; while some are obvious on casual inspection, others require some study to pick out. Various aids have been developed, such as watermark fluid that wets the paper without damaging it.
Encoding an identifying code into digitized music, video, picture, or other file is known as a digital watermark.