Longman Dictionary of Contemporary English homepage

watermark

noun
     
wa‧ter‧mark [countable]
1TCN 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'.
2 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

high watermark

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

high/low watermark

American English a line showing the highest or lowest levels of the sea [= tide-mark British English]

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