noun
A translucent design impressed on the surface of paper and visible when the paper is held to the light.
(by extension) A logo superimposed on a digital image, a television broadcast, etc.
(computing) 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.
(graphical user interface) A default hint caption displayed in a text box prior to the user inputting any text.
A mark indicating the level to which water has risen.
verb
(transitive) To mark paper with a watermark.
(transitive) To mark a datafile with a digital watermark.