Tuned (See also: Polished, Tweaked)
A game mechanic can be considered tuned when it correctly constrains the player experience to have the desired effect
The opening four bars of “Smoke on the Water” by Deep Purple are perhaps the best example of the power chord. Nothing screams sex, drugs and rock & roll like these iconic sounds.
But if you play them even a tiny bit incorrectly, you’ll have a discordant mess. Instead of the desired effect (impressing all the girls at the party) you will achieve the exact opposite. In the same way, game mechanics must be precisely tuned to insure they work together to produce the desired experience and prevent undesired ones.
Unlike balance, which must be considered across the entire community for the life of the game, a game that is tuned for one individual is probably tuned for most players. That is because the tuning process constrains the entire possibility space, not a specific experience. As it eliminates poor experiences and emphasizes or rewards the desired ones, the differences between player skill levels and choices are automatically included.