stimulus

Stimulus is any event or situation that elicits a response in an organism or system. In biology and neuroscience, a stimulus is detected by sensory receptors and converted into neural signals in a process called transduction. The strength and quality of a stimulus are encoded through mechanisms such as firing rate, receptor recruitment, and population coding. Receptors are often modality-specific, and responses depend on thresholds, including absolute thresholds for detection and just noticeable differences for discrimination. Sensory adaptation can reduce responses to constant stimuli over time.

In psychology and behavior, stimuli can be external (light, sound, touch, odor) or internal (pain, thoughts, arousal). They play a central role in learning and behavior: stimulus–response theories, stimulus control in operant conditioning, and classical conditioning where a neutral stimulus becomes predictive of an outcome.

In economics, stimulus refers to policy measures aimed at boosting economic activity, for example fiscal stimulus such as increased government spending or tax relief, and monetary stimulus such as lower interest rates or quantitative easing. The goal is to raise demand, output, and employment, while debates focus on timing, magnitude, effectiveness, and potential inflation or debt effects.

The word derives from Latin stimulus meaning "goad" or "spur," reflecting its role in provoking responses across disciplines.