The creation of balance to prevent a game from being too easy or too difficult for players. Also applies to competitive balance.
Balance is the art of finding the window which houses good gameplay experiences, as sitting outside of it may make the game feel boringly easy, unfairly hard, or worse. Balance can apply to both singleplayer and multiplayer scenarios, between whether it represents the player versus the challenges of the game, or the balance of players in comparison to each other.
Questions under this tag deal with the principles of what factors can affect balance, how to ensure balance in your game, and what affordances can be made to shift the scales without making completely breaking them.