What do we call what an organism does in reaction to a stimulus?

The term that describes what an organism does in reaction to a stimulus is 'response.' This refers to the specific action or change in behavior that occurs as a direct result of the stimulus. A stimulus is something that provokes a change, and the response is the observable outcome of that stimulus.

For example, if a dog hears a loud noise, its response might be to bark or run away. This highlights that the response is a measurable and attributable action following the exposure to a stimulus.

While 'behavior' and 'reaction' are related concepts, they encompass a broader range of actions or changes, and are not exclusively tied to the stimulus-response relationship. 'Instinct' refers more specifically to innate behaviors that are not learned but rather are biologically programmed responses to certain stimuli. Understanding these definitions helps clarify why 'response' is the most accurate term for the action taken by an organism in reaction to a stimulus.