Definition:
The direct result of input; the immediate properties of avatar behavior caused by the player.
Avatar is used for lack of a more all-encompassing term for the in-game agent representing the player’s will. It is meant here to include the unseen “body” (regardless of that body’s physicality) of the player character in a first-person shooter, the cursor in a real-time strategy game, and the indicator denoting the current menu selection in a role-playing game.
A lack of input is here considered a form of input, including both intentional pauses within a sequence of commands and a protracted idle period.
The result of input and the properties of those results are two different things. The latter are important for mechanical texture, which will be discussed following elaboration upon and defense of this definition.
Illustrations:
Essentially, imagine Mario in a vacuum, so to speak. Picture a barren world that consists of nothing but a straight line representing the ground, and play Mario. Everything possible within this scenario represents mechanics, and the qualities of these mechanics can be said to include momentum, gravity, and so on.
A mechanic represents an ideally replicable cause–effect relationship. Every time a command or sequence of commands is given, the same mechanical result occurs independent of non-player influences such as situational elements—the series of button presses that causes Mario to run and jump will give a different result if a pipe is in his way. Whenever the same command yields two different results, something extra-mechanical is interfering.