Any event, object, or situation that elicits a response from an organism counts as a stimulus. In psychological research the term is defined functionally by its effect on behavior or experience.

Stimuli may be external or internal, simple or complex, neutral or already meaningful. In conditioning, stimuli acquire new powers through association. In perception, stimuli provide the energy that sensory systems transduce. Discriminative stimuli signal when a particular behavior is likely to be reinforced.

Controlling stimuli is a basic experimental strategy: present, withhold, or vary them and observe the consequences for behavior and physiology. In everyday life the environment continually supplies stimuli that shape attention, emotion, and action. Understanding which stimuli control a behavior is often the key to changing it.

• Event or object that evokes a response
• Defined by its functional effect
• Central to conditioning and perception
• Can be manipulated to study and change behavior