In psychology, a stimulus is any object or event that elicits a sensory or behavioral response in an organism. In this context, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs).
In perceptual psychology, a stimulus is an energy change (e.g., light or sound) which is registered by the senses (e.g., vision, hearing, taste, etc.) and constitutes the basis for perception.
In behavioral psychology (i.e., classical and operant conditioning), a stimulus constitutes the basis for behavior. The stimulus–response model emphasizes the relation between stimulus and behavior rather than an animal's internal processes (i.e., in the nervous system).
In experimental psychology, a stimulus is the event or object to which a response is measured. Thus, not everything that is presented to participants qualifies as stimulus. For example, a cross mark at the center of a screen is not said to be a stimulus, because it merely serves to center participants' gaze on the screen. Also, it is uncommon to refer to longer events (e.g. the Trier social stress test) as a stimulus, even if a response to such an event is measured.
The study of the stimulus in psychology began with experiments in the 18th century. In the second half of the 19th century, the term stimulus was coined in psychophysics by defining the field as the "scientific study of the relation between stimulus and sensation". This may have led James J. Gibson to conclude that "whatever could be controlled by an experimenter and applied to an observer could be thought of as a stimulus" in early psychological studies with humans, while around the same time, the term stimulus described anything eliciting a reflex in animal research.
The stimulus concept was essential to behaviorism and behavioral theories of B. F. Skinner and Ivan Pavlov in particular. Within such a framework several kinds of stimuli have been distinguished.