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.
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.”
Two branches
做進紙或檔案裡、證明來歷的記號
這一群抓的是核心畫面的前半段：記號不是印在表面，而是做進東西本體裡，所以平常看不見，也很難仿造。正因為難仿造，它才拿來回答「這是誰的、是不是真的」。
noun · B2
A design or pattern made into paper while the paper is being made, especially on banknotes, stamps and official documents. You usually cannot see it when the paper lies flat; you have to hold it up to a light.
浮水印
水留下的那條線
這一群抓的是核心畫面的後半段：水走了，痕跡留下來，而那條痕跡記錄了水曾經到過的高度。從真實的牆面再走一步，就變成講一件事最高到過哪裡。
noun · B2
A line or stain left on a wall, a floor or a piece of furniture that shows how high water once rose, or where water has dried on the surface.
水痕；水漬