The expected result of the player's attempt to change the game world by interacting with some object in it. For example, a design system may define a door object, so that a designer need not write code that describes how the door responds to the player's attempts to open, close, or walk through it.
The default behavior typically includes refusal messages that describe what happens when a player tries to misuse the object (e.g. opening a door that is already open, or walking through a door that is still closed).
Default behavior is simply that -- a default; designers are free to modify the behavior of any object, so that opening a door triggers a bucket of water to fall, or kissing a frog turns it into a handsome prince.