The pairing of a stimulus, anything present in the environment which the subject (or learner) can feel through his or her senses, and a response, any behavior on the part of the subject, forms the principal mechanism of learning in behaviorist psychology.
Stimulus-response pairs are central to both Pavlov's classical conditioning and Skinner's operant conditioning. The father of behaviorism, John Watson, declared his aim was to eliminate "the terms consciousness, mental states, mind, [. . .] and the like" and conduct psychology "in terms of stimulus and response, in terms of habit formation."
Psychologists speak of stimulus-response pairs or units (S-R units).