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 page content — text, lines, images — and page content does nothing when it is clicked. Anything somebody can click, tick or type into therefore sits above the page content, at its own place on the page, with its own behaviour attached.
It usually carries an appearance too — an outline, a background, a tick — and the reading software shows that appearance while nothing is happening to the annotation.
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 defined by the rectangle it covers and not by the words underneath: the words are page content, and the rectangle is what responds to the click.
Because it is laid over what is already there, it can be added once the page content is created — over a table that turned out longer than expected, or on every page of the finished document, however many there turned out to be.
Where it comes from, and why
Notes stuck on a page by somebody reviewing it and links to follow were both there from the first version of the format. Form boxes were built on the same mechanism later, because they need the same thing — a place on the page and a behaviour.
The name stuck, which is why a button on a PDF is still called by a word that sounds like a comment in a margin.