Wiktionary
phthalazine
n. (context organic compound English) A bicyclic heterocycle consisting of a benzene ring fused to pyridazine
Wikipedia
Phthalazine
Phthalazine, also called benzo-orthodiazine or benzopyridazine, is a heterocyclic organic compound with the molecular formula CHN. It is isomeric with other naphthyridines including quinoxaline, cinnoline and quinazoline.