caf·feine also caf·fein 
(kă-fēn, kăfēn′)
(kă-fēn, kăfēn′)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.]
- on the ropes
- on the run
- on the same wavelength
- on-the-scene
- on the shelf
- on the side
- on the sly
- on the spectrum
- on the spot
- on the square
- on the street
- on the strength of
- on the string
- on the surface
- on the table
- on the take
- on the tapis
- on the tip of (one's) tongue
- on the tip of one's tongue
- on the tip of tongue
- on the town
- on the up and up
- on the up-and-up and up
- on the up-and-up/up and up
- on the upgrade