The Collaborative International Dictionary
caffein \caf*fe"in\, Caffeine \Caf*fe"ine\, n. [Cf. F. caf['e]ine. See Coffee.] (Chem.) A white, bitter, crystallizable substance, found in coffee and tea. It is identical with the alkaloid theine from tea leaves, and with guaranine from guarana. It is responsible for most of the stimulating effect of coffee or tea.
Wiktionary
n. caffeine when present in tea
Usage examples of "theine".
The chemical principle of tea, namely theine, was subsequently tried and produced no effect.
The French botanist Weddell thought that coca contained some kind of mild stimulant such as theine, or perhaps even caffeine.
In the nominative case,an erroneous accent over the antepenult indicatesthat you take the last letter of the word immediately following theone wrongly accented, and so forth.