watermark

Short definition from Wiktionary

Noun

1. 1
   A translucent design impressed on the surface of paper and visible when the paper is held to the light.
2. 2
   A logo superimposed on a digital image, a television broadcast, etc.
3. 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. 4
   A default hint caption displayed in a text box prior to the user inputting any text.
5. 5
   A mark indicating the level to which water has risen.

Verb

1. 1
   To mark paper with a watermark.
2. 2
   To mark a datafile with a digital watermark.