I know I've mastered the ability to walk, talk and read. There's nothing much left to learn in these fields, and I can do it all with complete ease. Doesn't mean I never stub my toe, slur a few words, or stumble on a sentence now and then, but I've reached a point where it's completely causal. I'm not an olympic sprinter, or a public speaker, or a speed reader -- those are all entirely separate skills -- but what I learned is plenty to get through life.
I never forget that the martial arts are called an art, whether or not that's an Eastern interpretation, but in the West this does have a purpose. An art couldn't be something you perfect. Music, painting and sculpting all have near limitless potential, but stages of development that you would have to "master" to get better.
A black belt, as we've explained before on here, is about having a firm understanding of the fundamentals, or "All basic movements and techniques, can be applied with extended force and proper application in basic combination." The key words here would be "all basic movements and techniques" and "extended force and proper application". I would say there is a mastery of having LEARNED the requirements and KNOW how to put them to use, just not anything further from the basics.
I think people go a little too far with the word, using it mostly erroneously, but the intent of comfortably and casually using what you've learned is what they're all implying. As for the martial arts as a whole, that I believe is impossible, especially as an art.