What do you mean, "what term"? We're talking about the semantics of a combination of language features to be unspecified or undefined. That expression in that language is the "term". That's why it's called a language. The syntax has semantics -- the expressions have meaning. An expression that is "undefined" has no accepted meaning.

I think you're stretching a bit to arrive at your stated premise. Perhaps my understanding of "behavior" and "undefined" by themselves differ from yours, because I never had a problem understanding that "undefined behavior" meant there was no prescriptive meaning assigned to a certain syntactical construct. I didn't have to learn it by rote, and it was clearly logical to me.

As I said before, though, if you find it too idiomatic to be useful, then don't use it. Just don't assume that because you say it can't be understood without a glossary that people will accept that statement based on you making it.