"Abstract"! That's it! I'm not a native English speaker, and I was wondering what already existing (and more positive) term could replace "TL;DR". I had simply thought of "summary", but "abstract" seems more accurate. I guess people avoid it not to look pretentious... though the semicolon in "TL;DR" could also be perceived as such.

What DO you put an abstract on then? 'prose' is everything written but poetry.