any of a class of organic compounds of the aromatic heterocyclic series characterized by a double-ring structure composed of a benzene and a pyridine ring fused at two adjacent carbon atoms. The benzene ring contains six carbon atoms, while the pyridine ring contains five carbon atoms and a nitrogen atom. The simplest member of the quinoline family is quinoline itself, a compound with molecular structure C₉H₇N.

Quinoline is used principally for the manufacture of nicotinic acid, which prevents pellagra in humans, and other chemicals. Several methods are known for its preparation, and production of synthetic quinoline exceeds that from coal tar.

Several alkaloids (alkaline organic compounds produced in plants) are members of the quinoline family; these include quinine and other derivatives from the cinchona tree. The antimalarial drugs chloroquine and amodiaquin are synthetic quinoline compounds, as are dibucaine hydrochloride, a long-acting local anesthetic, and cyanine, the oldest of an important class of dyes.