When I sat down to type out my thoughts on Mastery last time, I wasn't sure that I was going to continue. Not because I said all there was to be said (certainly not), but because I was quite certain that whatever I said next (if anything) ought to be worth saying. This is far from an obvious conclusion, for people often ramble on about a subject far longer than necessary, in much the same fashion as I am currently doing. However, this is not without purpose, and serves as an example of what Mastery is not.
Primarily, Mastery is not superfluous. It does no more than is necessary. It does not write a paragraph when a sentence or a single word will do. Mastery is not just the ability to know what is necessary, but the ability to do that very thing at will. In fact, they are one and the same to the person in possession of Mastery.
That leads me to the second thing Mastery is not: contemplative (with exception). It is possible to reason over time what is the necessary action and then complete that action based upon the outcome of your contemplation, but that is not Mastery. If asked a question, a Master mathematician does not ponder the problem at length and then give sufficient answer. He merely answers, because the rules of the mathematics are so deeply ingrained in him, he cannot help it. It is the same as tossing a ball to someone and seeing them catch it without thought. They did not calculate the trajectory and position themselves in such a fashion to be able to catch the ball-- they just caught it. To be fair, I'm certain there are Master mathematicians who use calculators or must consider a problem shortly before answering extraordinarily complex problems. Indeed, things of sufficient difficulty can force any Master to struggle or even fail. This may seem like a contradiction, but only if you think of Mastery in completely finite and blanketing terms. Depending on the subject or the method, Mastery may mean different things. I use such a complex thing as math to lay out my next point.
Mastery is not definitive. What I mean here is that Mastery can change over time. In the scope of an overarching concept such as math, which is constantly evolving, it is only possible to attain Mastery if one has Mastery over the concept's parts. Math can be as basic as adding and subtracting or simple algebra or as complex as is necessary to explain the most detailed of physical or non-physical representations. This then means that if you can count to 100 without thought, you've gained Mastery of the ability to count to 100. It seems overly simple, I realize, but it is necessary to think in these terms because of the sheer vastness of the concept of mathematics (granted, math is not the only thing that is horribly, horribly complex). To have Mastery of such a thing, one must be expected to be enormously skilled at a variety of different mathematical procedures, often without having to really think about them. The other side of Mastery not being definitive is that, Masters often expand the field in which they specialize. The Master mathematicians or martial artists of 500 years ago are not the same, for things that exist today did not exist at that time (though are the product of those men and women's pursuits). In this way, Mastery is not definitive, because it expands and develops as it goes.
This post and its predecessor are not the be-all, end-all word on Mastery. There is much more that has can be said and has been said by far better men than myself. As far as your own journeys go, I hope I have been able to shed some light to whatever paths you are following.