A conditioned stimulus is an originally neutral stimulus that, after association with an unconditioned stimulus, comes to trigger a conditioned response. It acquires its power through learning.

In the standard paradigm the conditioned stimulus is presented slightly before the unconditioned stimulus so that it becomes a reliable predictor. A tone that precedes food, a light that precedes a mild shock, or a particular flavor that precedes illness can all become conditioned stimuli. The timing, intensity, and reliability of the pairing determine how effectively the association is formed. Once conditioned, the stimulus can be used in further experiments—higher-order conditioning, sensory preconditioning, or extinction—to map the rules of associative learning.

Conditioned stimuli are ubiquitous outside the laboratory. Brand logos, medical settings, and social cues all function as conditioned stimuli that elicit emotional and physiological responses on the basis of past pairings. Advertising and clinical treatments both exploit this capacity of the nervous system to treat predictive signals as significant.

A conditioned stimulus is a learned messenger: it announces that something important is about to occur.