Think of watermark as a mark that is part of the thing itself, not printed on top of it, and that shows up only when the conditions are right. Hold the paper up and light passes through it, and a shape appears. Wait for the water to drop, and the line it reached is still on the wall. Once you can see the mark, it tells you something true: how high the water came, or where this sheet, this photo, this file came from.
From the word’s history
The evidence we have records the word only as water + mark. That much is documented. How the papermaking use grew out of it is not stated in this evidence, so the link between the two groups below is offered as a way of seeing them together, not as a claim about history.
我們手上的資料只記了一句：這個字來自 water（水）加 mark（記號）。這一點是有出處的。至於造紙的用法是怎麼長出來的，資料裡沒有說，所以下面兩群意思之間的連結只是一個幫助理解的看法，不是歷史上的推論。
Wiktionary (kaikki dump, source_ref kaikki:3f6f69a190036ae4:837875815:26092), etymology_text: “From water + mark.”
登入就可以看每一支怎麼連回核心畫面。