A Grandmaster of Chess has achieved a level of proficiency. This mastery is knowledge and ability demonstrated at a very high level.
Chess is great, it's so damn fair.
What doesn't seem fair is that anyone can claim to be a Master of Something without having to demonstrate a single thing.
Mastery appears to be knowledge and ability demonstrated at the highest possible level, hopefully recognized by peers.
Now we can all sleep easier, knowing the question has been mastered.