Caffeine is a bitter white alkaloid that acts as a psychoactive stimulant drug and a mild diuretic (speeds up urine production).
Caffeine was discovered by a German chemist, Friedrich Ferdinand Runge, in 1819. He coined the term "kaffein", a chemical compound in coffee, which in English became caffeine.
Caffeine is also called guaranine when found in guarana, mateine when found in mate, and theine when found in tea; all of these names are synonyms for the same chemical compound.