What is "mastery" in a mathematical topic?

1. I don't think fighting for the definition of a word makes most sense. After all, we could consider Robbie mastery versus Kahn Academy mastery (as different attributes).

2. For what it's worth, the Kahn Academy sense of mastery is a rather normal one. The idea comes from the theory of building block automaticity. That you have skills down to the sense that you can use them without struggle. Our minds are not silicon, they have very limited RAM. So if you struggle on basic reading, it becomes a chore to do the reading and then do higher processing. Similarly, if you struggle on basic manipulations, than new concepts in physics or math are hard to work on. Consider the issues for a kid working in calculus who is weak in trig or doesn't have an iconic recall of (-b +/-sqrt(bsq-4ac)/2a). [Own goal on me, if I flubbed that formula!]

Here is an example of Richard Feynman discussing mastery in the sense of manipulation skill:

So, this guy comes into my office and asks me to try to make everything straight that I taught him, and this is the best I can do. The problem is to try to explain the stuff that was being taught. So I start, now, with the review. I would tell this guy, “The first thing you must learn is the mathematics. And that involves, first, calculus. And in calculus, differentiation.”

Now, mathematics is a beautiful subject, and has its ins and outs, too, but we’re trying to figure out what the minimum amount we have to learn for physics purposes are. So the attitude that’s taken here is a “disrespectful” one towards the mathematics, for sheer efficiency only; I’m not trying to undo mathematics.

What we have to do is to learn to differentiate like we know how much is 3 and 5, or how much is 5 times 7, because that kind of work is involved so often that it’s good not to be confounded by it. When you write something down, you should be able to immediately differentiate it without even thinking about it, and without making any mistakes. You’ll find you need to do this operation all the time—not only in physics, but in all the sciences. Therefore differentiation is like the arithmetic you had to learn before you could learn algebra.

Incidentally, the same goes for algebra: there’s a lot of algebra. We are assuming that you can do algebra in your sleep, upside down, without making a mistake. We know it isn’t true, so you should also practice algebra: write yourself a lot of expressions, practice them, and don’t make any errors. Errors in algebra, differentiation, and integration are only nonsense; they’re things that just annoy the physics, and annoy your mind while you’re trying to analyze something. You should be able to do calculations as quickly as possible, and with a minimum of errors. That requires nothing but rote practice—that’s the only way to do it. It’s like making yourself a multiplication table, like you did in elementary school: they’d put a bunch of numbers on the board, and you’d go: “This times that, this times that,” and so on—Bing! Bing! Bing!

3. I would also suggest that instead of debating the usage of the term mastery, you could also differentiate the subject. In other words, if I master Thomas Finney calculus. (I literally and not how millennials use that word did every homework problem in that text and scored perfect tests throughout high school and as far as I could tell on the AP exam.) Now I would say that was automaticity and ease of use and Kahn-Feymnanian mastery OF the 1980s AP calculus class. Of course if you want to say there should be some OTHER calculus course (with a bunch of real analysis thrown into the frosh calc course instead of waiting until junior year), then fine. But you're really debating the subject, not the concept of mastery. After all mastering high school geometry certainly involved lots of proofs since that course was mostly proving geometric propositions (was actually weak on mensuration practice, I thought.)

4. Also, I think there are some reasonable differences in the depth of mastery expected of a student and of a teacher. Both should have acquired automaticity at doing the homework drill problems. But the teacher actually benefits from knowing things not really even PART of the course for the student. Advanced courses that use the skills from the course in discussion. The history of the development of the subject. And applications. But it's not reasonable to expect the same thing of the student. He just needs to master the subject itself. Not a bunch of related matters outside the scope.

5. I don't think nitpicking a mistake in one of his videos is relevant to the concept of student mastery and/or the preferred definition/usage of the term. They are really not connected.

The Kumon Math Program defines mastery as

The ability to apply a skill or concept with the accuracy, speed and confidence to demonstrate a total command of the material. Mastery means that a skill or concept has become second nature and will be permanently retained. Mastery is the prerequisite for advancement and the goal of Kumon study. [source]

Also,

Speed + Accuracy = Mastery
Material should be completed with a perfect score within a prescribed period of time. [source]

This "prescribed period of time" is called the Standard Completion Time, which is

A prescribed number of minutes to complete a particular worksheet with a perfect (or near perfect) score. Kumon determines-and continually reevaluates-SCT based on the actual performance of tens of thousands of students. Successfully completing worksheets within the SCT demonstrates that the student has mastered the material and is ready to move on. [source]

In practice, "near perfect" usually means "at least 90%."