caf·feine also caf·fein (kā-fēn', kāf'ēn', kāf'ē-ĭn) n. A bitter white alkaloid, C8H10N4O2, often derived from tea or coffee and used in medicine chiefly as a mild stimulant and to treat certain kinds of headache.
[German Kaffein (from Kaffee, coffee) or French caféine, both from French café, coffee; see café.] caf'fein·at'ed (kāf'ə-nā'tĭd) adj.