Caffeine (also spelled caffein) is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant. Caffeine was isolated from coffee in 1820 by a German chemist, [Friedlieb Ferdinand Runge , and in 1821 by French chemists working independently; viz., by Pierre Jean Robiquet , and by Pierre Joseph Pelletier  and Joseph Bienaimé Caventou . It was Pelletier, noting that the drug had been isolated from coffee (French: café), who coined the word "cafeine", which became the English word "caffeine".
Caffeine content in coffee varies widely depending on the type of coffee bean and the method of preparation used; even beans within a given bush can show variations in concentration. In general, one serving of coffee ranges from 80–100 milligrams, for a single shot (30 milliliters) of arabica-variety, to approximately 100–125 milligrams for a cup (120 milliliters) of drip coffee.
In general, dark-roast coffee has less caffeine than lighter roasts because the roasting process reduces the bean's caffeine content.