An alkaloid, C₈H₁₀N₄O₂, found naturally in tea and coffee plants which acts as a mild stimulant on the central nervous system.
"Caffeine is an alkaloid found naturally in tea and coffee plants that acts as a mild stimulant on the central nervous system."
In plain English: Caffeine is a natural stimulant found in coffee, tea, and soda that makes you feel more awake and alert.
"She added caffeine to her morning coffee to help her wake up faster."
Usage: Avoid using "caffeine" to refer to coffee or tea itself; specify the beverage if you mean the drink containing the substance. The term is also often misspelled with an extra 'f' (caffiene), so ensure correct spelling in formal writing.
The word caffeine comes from French caféine and German Caffein, both derived directly from the words for coffee in those languages. It was formed by adding the suffix "-ine" to indicate a chemical substance related to coffee.