Things that cause a reaction or response in a living organism, like a bright light making you squint or a loud noise making you jump.
From Latin 'stimulus' meaning 'goad' or 'spur,' originally referring to a stick used to drive cattle. The plural form 'stimuli' follows Latin pluralization rules and entered English in the 1600s when scientists began studying how organisms respond to external factors.
Your brain processes thousands of stimuli every second—sounds, smells, textures—but your conscious mind only pays attention to a tiny fraction, which is why you can suddenly 'notice' your shirt only when someone points it out!
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