A somewhat ambiguous term for the way an object in the game world typically reacts to player input. This reaction may describe the results of default behavior that affects the game world in some way ("You unlock the steel grate."), or it may be a simple refusal message ("You can't do that here." or "Boy, that was fun!") that does not have any effect on the game world.
The first kind of default response occurs when the player requests to perform a common action like TAKE LANTERN. Unless the designer has a good reason to interrupt the default behavior (e.g. the lantern is too hot to touch, or is currently being held by a nasty pirate), the game would ordinarily move the lantern to the player's inventory and then print a confirmation message such as "Taken." A design system may provide special kinds of objects with more complex default behaviors that describe their effect upon the simulated world. For instance, an object defined as a door would have meaningful default responses to commands like OPEN or ENTER. The default behaviors change based on whether an object is defined as alive, a container, or openable/closable -- or all of the above, such as a Venus flytrap.
The other kind of default response is stimulated by standard commands such as JUMP, CLIMB, and SWIM -- actions that are only occasionally useful. For example, entering the command SWIM might provoke the default response "There is nothing to swim in here." If a room description should happen to mention a swimming pool, then the "nothing to swim in" message would be inappropriate (see mimesis). In such a case, a conscientious designer would change the plain-vanilla, makes-sense-just-about-anywhere response to a situation-specific refusal message: "And ruin your prom dress? As if!"