the.com/watermark

means A faint identifying mark built into paper or an image to show its source or prove it's genuine.

from From water + mark — born in papermaking, where a wire design pressed into the pulp mold left a thinner patch in the drying sheet, visible only when held up to light, as if the water itself had drawn a picture. Centuries later the word slid naturally onto digital images and documents, keeping its original job: a mark made from the inside, not stuck on after.