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A stimulus is something that causes a physiological response:
Stimulation
Stimulus (physiology), something external that influences an activity
Stimulus (psychology), a concept in behaviorism and perception
It may also refer to:
Input to a system in other fields
Economic stimulus
For government spending as stimulus, see Fiscal policy
For an increase in money designed to speed growth, see Monetary policy
For general information about economic stimulus, see Stimulus (economic)