What is a water mark on a stamp?

Answer 1

A figure or design impressed in some paper during manufacture, visible when the paper is held to the light.

watermark is used to protect images,video,audio or documents.

sometimes they add visible watermark but they also use invisible watermarks to protect the copyright ;)

Answer 2

When paper is made the manufacturer uses a large screen to smooth out the paper pulp. They often put their logo on the screen. This creates a slightly thinner layer of pulp in that area. The logo can be seen in the finished paper by holding the paper up to the light. Many governments used watermarked paper to help avoid the creation of counterfeit stamps. Errors can occur when the watermarked paper is fed into the presses upside down.

Answer 3

A watermark on a stamp, as well as any other paper can be used to identify size, quality, thickness or density and in some cases, security as in bank notes and passports. In stamp collecting, identical stamps may have different water marks and this is important to a specialist collecter