layer
A layer is a named group of things drawn on a page that whoever opens the file can switch on and off.
In more detail
Think of tracing paper laid over a plan: the plan underneath does not change, and lifting one sheet takes its lines away with it. A page can carry several, and each is named so a reader can tell them apart.
A layer says how it starts — showing or hidden — and can be told to stay off the printer while showing on screen, or the other way round.
What it does on a real document
A drawing holds its measurements on one and its notes on another, so one file serves the workshop and the customer. A contract holds its clause numbers on one that only prints.
Everything drawn on a page can be put on one: a block of text, a table, a picture, a shape. An item that names no layer belongs to the page itself and is always there.
Where it comes from, and why
Drawing offices worked this way on paper long before any of it was on a screen, and the drawing programs that followed kept both the idea and the word.
PDF gained it once files started being used as the drawing itself rather than as a picture of one, and it is the reason a single file can carry several readings of one sheet.
See it on a finished document
The rules behind this word, and the texts they are read from