Xanthine \Xan"thine\, n. Also Xanthin \Xan"thin\ . [Gr. xanqo`s yellow.]

(Physiol. Chem.) A white microcrystalline nitrogenous compound, C5H4O2N4, present in muscle tissue, in the liver, spleen, pancreas, and other organs, and also in urine (in small quantities) and some urinary calculi, and in the juices of certain plants; -- so called because it leaves a yellow residue when evaporated to dryness with nitric acid. Xanthine is closely related to uric acid.

Source: Webster's Revised Unabridged Dictionary (1913)