The Collaborative International Dictionary of English v.0.48:

Juglone \Ju"glone\, n. [L. juglans the walnut + -one.] (Chem.)
   A yellow crystalline substance (C10H6O3) resembling
   quinone, extracted from green shucks of the walnut (Juglans
   regia); -- called also nucin. Chemically, it is
   5-hydroxy-1,4-naphthalenedione.
   [1913 Webster]