pyrazolone

from The Century Dictionary.

• noun A crystalline base, , made by the action of hydrazine sulphate on sodium-formyl-acetic ester. It melts at 165° C. Its most important derivative is antipyrin, which is 1-phenyl-2,3-dimethyl-pyrazolone.

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

• noun organic chemistry A five-membered unsaturated heterocyclic ketone (or lactam), derived from pyrazole, that is the basis of several pharmaceuticals