Caffeine

a naturally-occurring chemical substance that is found in tea leaves, coffee beans and other plants. It acts as a weak stimulant on the central nervous system so after drinking a cup of coffee or tea, a person may feel more alert.

Taken from Dictionary of Science

____________________________________________________________

Caffeine is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant drug. 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.

Taken from Wikipedia