phthalazine

from The Century Dictionary.

• noun A crystalline base, C₈H₆N₂, made by boiling phthalic aldehyde with an aqueous solution of hydrazine. It melts at 90–91° C.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun organic chemistry A bicyclic heterocycle consisting of a benzene ring fused to pyridazine