Something that excites your interest, creates energy, or causes a response; to make something happen or increase activity in something.
From Latin 'stimulare,' from 'stimulus' (a goad or pointed stick used to drive animals). The sense of 'exciting mentally' developed in the 1600s-1700s.
Caffeine is a stimulant that blocks adenosine receptors in your brain—the very word 'stimulus' comes from ancient cattle goads, so literally you're being 'poked' awake by your coffee!
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