The general term in publishing for boxes, usually of text, set out from the rest of the text, is boxout, also box-out or occasionally box out:
A piece of text written to accompany a larger text and printed in a separate area of the page.
However, that's not usually used if there's a table or a graphic in it. Then it's called a table or a figure. In scientific typesetting circles, the catchall term for all three is float - whether that was originally the term and LaTeX used it, or LaTeX introduced it and it caught on, I don't know. I'm not aware of a catchall term for such things that is used generally by everyone.
Oh, and if such things are printed in the margins, they might be referred to as marginalia, a term originally used for additions made by readers by hand, as they read the text, but that I have seen used to refer to things deliberately printed in margins, especially of self-teaching books.
It's worth noting, though, that "not a word" is a flexible concept in English. Not as much as it is in German, perhaps, but flexible. There's no official list of words, not even one that has supposed status but limited real influence - there's just no official list. At all. Words that get used are words, and infobox has appeared in the Google Books corpus since the late 80s - albeit in very small numbers.