Watermark

What is Watermark?

Watermark is (noun) 1. a hidden mark in paper, usually put there to prove that the paper is legal or official If you hold a banknote up to the light you will see the watermark. 2. a line showing the level water has reached On the wall by the river you can see various watermarks showing the level of floods in different years.

 

source: Easier English, Student Dictionary Upper Intermediate Level