Digital watermark
A digital watermark is a kind of digital signature, usually in the form of a subtle modification to the main content of an image, audio, or video file. To put it another way, a digital watermark stores metadata inside the main content of a file, instead of separately. There are many different reasons for using a watermark, but the main point is to make it difficult to remove the metadata.
A digital watermark could be anything from a clearly visible overlayed semi-transparent copyright notice, to a steganographic technique that makes the signature virtually undetectable, or anything in between. It could be a simple notice of what software created the file, or it could contain detailed tracking information intended to help figure out who created the file.
Calling something a "watermark" usually implies that it is designed to survive at least minor modifications to the document.
Similar to bar codes, some kinds of watermarks store digital data in physical documents. See Watermark (physical).
