watermark

noun

1. A translucent design impressed on the surface of paper and visible when the paper is held to the light.
2. (broadly)A logo superimposed on a digital image, a television broadcast, etc.
3. A value stored in a datafile to ensure its integrity, so that if the file's contents are changed then the watermark will no longer match the contents.
4. A default hint caption displayed in a text box prior to the user inputting any text.
5. A mark indicating the level to which water has risen.

verb

1. (transitive)To mark paper with a watermark.
2. (transitive)To mark a datafile with a digital watermark.