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In physiology, a stimulus (plural: stimuli) is something external that elicits or influences a physiological or psychological activity or response.

In psychology, anything effectively inpinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body.

In general, a stimulus may affect other things than living creatures; for example, one speaks also of economic stimulus, or stimulating a discussion.

See also response, stimulus-response model.