Game mastery is the player’s sense that the game rewards understanding, timing, planning, and execution in a dependable way. It depends on players being able to perceive the difference between better and worse play, which is why it connects closely to perceivable-margins, fun-as-learning, and systemic-depth-elegance. If balancing systems or opaque randomness erase too much of that difference, mastery becomes harder to feel even if the game remains technically fair (Björk & Holopainen, Patterns in Game Design, see source-patterns-in-game-design).
Related: perceivable-margins, fun-as-learning, systemic-depth-elegance, game-balance