When speaking of digital data, a watermark is a type of information that is "hidden" among the data. This is mostly used with pictures and sound data.
Watermarks serve mostly only to identify the picture or sound; The artist can assign any watermark to a picture, containing (say) his name and your customer number, so that if someone copies the picture from you, he knows you leaked it. (Most of the image watermarks only record the artist, but sound watermarks such as SDMI record also the recipient. Caveat emptor.)
Unfortunately, watermarks are as of yet not a good way to put information into data; mostly for two reasons:
- None of the watermark algorithms can withstand tampering. Change image shades a bit, *poof* goes the watermark. Crop the image, *poof* goes the watermark. Save in lossy format - *bang*, now you killed it dead!
And so on. (For more proof of this, see the results of "stirmark" research.)
- Watermarks tend to lower signal quality (or make pictures look worse). Sometimes, the watermarks are very audible or visible!
See also steganography.