Pyr"i*dine (?), n. [From Gr. &?; fire.] (Physiol. Chem.) A nitrogenous base, C₅H₅N, obtained from the distillation of bone oil or coal tar, and by the decomposition of certain alkaloids, as a colorless liquid with a peculiar pungent odor. It is the nucleus of a large number of organic substances, among which several vegetable alkaloids, as nicotine and certain of the ptomaïnes, may be mentioned. See Lutidine.

• (chemistry) Any of a class of aromatic heterocyclic compounds containing a ring of five carbon atoms and an nitrogen atom; especially the simplest one, C5H5N.