In psychology, a stimulus is any object or event that elicits a sensory or behavioral response in an organism.
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.
In behavioral psychology
The concept stimulus was essential to behaviorism and the behavioral theory of B. F. Skinner in particular. Within such a framework several kinds of stimuli have been distinguished (see also classical conditioning):
An eliciting stimulus was defined as a stimulus that precedes a certain behavior and thus causes a response. A discriminative stimulus in contrast increases the probability of a response to occur, but does not necessarily elicit the response. A reinforcing stimulus usually denoted a stimulus delivered after the response has already occurred; in psychological experiments it was often delivered on purpose to reinforce the behavior. Emotional stimuli were regarded as not eliciting a response. Instead, they were thought to modify the strength or vigor with which a behavior is carried out.