Louis I. answered 04/20/19
Computer Science Instructor/Tutor: Real World and Academia Experienced
Well, I would agree with google - "well-definedness" is one of those made-up words that might get the right point across in casual conversation, but wouldn't pass a "white paper" review.
There are several general rules (or patters) we can observe and try to apply (like the ones described here - http://writingfix.com/PDFs/Writing_Tools/Adjectives_into_nouns.pdf), but I think the problem we have with "nounifying" ;-) well-defined in the mathematical context you describe is that there is no 1 single equivalent noun that gets the point across ....
I would argue you need at least 2: clarity and completeness (maybe throw in a 3rd - simplicity)
Imagine that you were trying to determine, by consensus, how well-defined something was: you might design a simple survey whereby responders would choose a 1-5 rating (5 being the best):
Clarity: 1 2 3 4 5
Completeness: 1 2 3 4 5
Simplicity: 1 2 3 4 5
And arguably, those 3 qualities would be a solid measure of how "well defined" something was ...
Hope this helps.
--Lou