(Followed by long explanation of Nomicon docs being likely imprecise.)
Sure, but all of that has nothing to do with the nature of UB. @alice went from "undefined behavior" to "defined behavior" as its complement, and that's what my comment referred to -- "undefined behavior" is not the opposite of "defined behavior", at least not if "defined behavior" is "behavior someone has defined". "undefined behavior" is not "the absence of something".
"Undefined behavior" is not a good choice of words for what it means these days, but well, we can't fix everything that C did wrong... if we also change terminology nobody will understand us. 
What it actually means is defined in the UCG glossary (though also see this PR that proposes to update that definition).
@CreepySkeleton I agree with (almost) everything you said, it just has little to do with the part of my post that you quoted. 
So who's going to open the Nomicon issue?