caf·feine also caf·fein (k?-f?n?, k?f??n?)n. A bitter white alkaloid, C8H10N4O2, found in certain plants such as cacao, coffee, kola, and tea, that stimulates the central nervous system and body metabolism and is used in medicine, usually in combination with other drugs, to relieve headaches and treat respiratory conditions in premature infants.[German Kaffein (from Kaffee, coffee) or French caféine, both from French café, coffee; see café.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.caffeine (?kæfi?n; ?kæf??i?n) or caffeinn (Elements & Compounds) a white crystalline bitter alkaloid responsible for the stimulant action of tea, coffee, and cocoa: a constituent of many tonics and analgesics. Formula: C8H10N4O2. See also xanthine2[C19: from German Kaffein, from Kaffee coffee]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014caf?feine (kæ?fin, ?kæf in) n. a white, crystalline, bitter alkaloid, C8H10N4O2, usu. derived from coffee or tea, used medicinally as a stimulant. [1820?30;