annotation
An annotation is something laid over a page rather than drawn into it: a clickable rectangle, a box to type in, a button.
In more detail
What is drawn into a page is ink, and ink does nothing when it is clicked. Anything a reader can interact with therefore sits above the page, at its own place on the sheet, with its own behaviour attached.
It usually has an appearance as well, which is what you see when nothing is happening: an outline, a background, a tick.
What it does on a real document
A clickable link is one of these, and so is every box of a form. That is why a link is described by the rectangle it covers rather than by the words underneath: the words are ink, and the rectangle is what answers to the click.
Because it is laid over what is already there, it can be placed after the drawing is done — over a table that turned out longer than expected, or on every page a document ended up having.
Where it comes from, and why
The first ones really were annotations in the ordinary sense: a note stuck on a page by somebody reviewing it. Links and form boxes were added to the same mechanism because they need the same thing — a place on the sheet and a behaviour.
The name stuck, which is why a button on a PDF is filed under a word that sounds like a comment in a margin.
See it on a finished document
The rules behind this word, and the texts they are read from
Glossary
- CMYK
- The four inks a printing press lays down: cyan, magenta, yellow and black. A screen mixes light instead, so a document going to a press states its colours in inks, not in screen colours.