3.5 DMG:
METAGAME THINKING
“I figure there’ll be a lever on the other side of the pit that deactivates the trap,” a player says to the others, “because the DM would never create a trap that we couldn’t deactivate somehow.” That’s an example of metagame thinking. Any time the players base their characters’ actions on logic that depends on the fact that they’re playing a game; they’re using metagame thinking. This behavior should always be discouraged, because it detracts from real roleplaying and spoils the suspension of disbelief.
Surprise your players by foiling metagame thinking. Suppose the other side of the pit has a lever, for example, but it’s rusted and useless. Keep your players on their toes, and don’t let them second-guess you. Tell them to think in terms of the game world, not in terms of you as the DM. In the game world, someone made the trap in the dungeon for a purpose. You have figured out the reason why the trap exists, and the PCs will need to do the same.
In short, when possible you should encourage the players to employ in-game logic. Confronted with the situation given above, an appropriate response from a clever character is “I figure there’ll be a lever on the other side of the pit that deactivates the trap, because the gnomes who constructed the trap must have a means to deactivate it.” In fact, this is wonderful—it shows smart thinking as well as respect for the verisimilitude of the game world.
4e DMG:
Metagame Thinking: Players get the best enjoyment when they preserve the willing suspension of disbelief. A roleplaying game’s premise is that it is an experience of fictional people in a fictional world.
Metagame thinking means thinking about the game as a game. It’s like a character in a movie knowing he’s in a movie and acting accordingly. “This
dragon must be a few levels higher than we are,” a player might say. “The DM wouldn’t throw such a tough monster at us!” Or you might hear, “The read-aloud text spent a lot of time on that door—let’s search it again!”
Discourage this by giving players a gentle verbal reminder: “But what do your characters think?” Or, you could curb metagame thinking by asking for Perception checks when there’s nothing to see or by setting up an encounter that is much higher level than the characters are. Just make sure to give them a way to avoid it or retreat.
5e DMG:
METAGAME THINKING
Metagame thinking means thinking about the game as a game. It's like when a character in a movie knows it's a movie and acts accordingly. For example, a player might say, "The DM wouldn't throw such a powerful monster at us!" or you might hear, "The read-aloud text spent a lot of time describing that door - let's search it again!"
Discourage metagame thinking by giving players a gentle reminder: "What do your characters think?" You can curb metagame thinking by setting up situations that will be difficult for the characters and that might require negotiation or retreat to survive.