When a player does something that causes a change in a game world, he has performed an action. Shooting a bullet, moving a piece, playing a card, ordering a unit to attack, casting a spell and twisting a tetromino are all kinds of action.
Actions are composed of verbs (individual inputs like clicking a button or pushing a controller stick). Depending on the game, one verb may lead to one action, or many verbs may lead to one action. Sometimes (particularly when the verb is something like holding a trigger for automatic fire) one verb can lead to many actions.
Actions which open loops are meaningful, whereas those that do not are busywork. While many games involve at least some degree of busywork, players prefer to take meaningful actions where possible. Such games are said to have high agency, whereas busywork-focused games have low agency.
The effects of actions need to be clear to the player, and also must feel fair. Actions that players can intuitively understand and find fair are natural actions, whereas those that feel unfair, are hard to understand or whose effects are difficult to perceive are arcane actions.
Actions which have many uses throughout a game are called extensible actions. Extensibility is the hallmark of good game design, as fewer types of action (though not too few) allows the play brain to understand the frame of the game better, leading to optimal tactics and strategy.
Finally, characters can also take actions. Enemies shooting at the player's doll are taking action, essentially as if they were other players.
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