a special design put onto paper, especially bank notes, that can only be seen when you hold it up to the light
bear/carry a watermark
The sheet bears the watermark '1836'.
a special mark contained in electronic documents, pictures, music etc that is used to stop people from copying them:
The card has a digital watermark detectable only to electronic cash dispensers.
3 especially American English
the high watermark of a particular process is its most successful time or achievement [= high point]
high watermark of
Reagan's presidency may prove to have been the high watermark of the US-Israeli alliance.
4 American English
a line showing the highest or lowest levels of the sea [= tide-mark British English]