caffeine

"Caffeine is known medically as trimethylxanthine, and the chemical formula is C8H10N4O2 (see this page for an image of the molecular structure). When isolated in pure form, caffeine is a white crystalline powder that tastes very bitter. The chief source of pure caffeine is the process of decaffeinating coffee and tea.

Medically, caffeine is useful as a cardiac stimulant and also as a mild diuretic (it increases urine production). Recreationally, it is used to provide a "boost of energy" or a feeling of heightened alertness. It's often used to stay awake longer - college students and drivers use it to stay awake late into the night. Many people feel as though they "cannot function" in the morning without a cup of coffee to provide caffeine and the boost it gives them.

Caffeine is an addictive drug. Among its many actions, it operates using the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain. On a spectrum, caffeine's effects are more mild than amphetamines, cocaine and heroin, but it is manipulating the same channels and that is one of the things that gives caffeine its addictive qualities. If you feel like you cannot function without it and must consume it every day, then you are addicted to caffeine."
                                                                                                                   --HowStuffWorks.com, by Marshall Brain

caffeine

\Caf*fe"ine\, n. [Cf. F. caf['e]ine. See Coffee.] (Chem.) A white, bitter, crystallizable substance, obtained from coffee. It is identical with the alkaloid theine from tea leaves, and with guaranine from guarana.

caffeine

n : a bitter alkaloid found in coffee and tea that is responsible for their stimulating effects [syn: caffein]