caffeine caf·feine or caf·fein (kā-fēn', kāf'ēn', kāf'ē-ĭn) n. A bitter white alkaloid often derived from tea or coffee and used chiefly as a mild stimulant and in the treatment of certain kinds of headache.
(kā-fēn') A bitter white alkaloid found in tea leaves, coffee beans, and various other plant parts. It is a mild stimulant. Caffeine is a xanthine and similar in structure to theobromine and theophylline. Chemical formula:C8H10N4O2.