- A yellowish- or brownish-red compound, HOC<sub>10</sub>H<sub>5</sub>O<sub>2</sub>. prepared by the oxidation of <em>a</em>-hydrojuglone from the green parts of the walnut-tree; <em>a</em>-hydroxynaphthoquinone, nucin, or regianin. It crystallizes in needles or prisms and melts at 151–154° C.
- A yellow crystalline substance (C10H6O3) resembling quinone, extracted from green shucks of the walnut (<spn>Juglans regia</spn>); -- called also <altname>nucin</altname>. Chemically, it is 5-hydroxy-1,4-naphthalenedione.
- An <xref>allelopathic</xref> <xref>aromatic compound</xref> found in the leaves, roots, husks, and bark of plants in the <xref>Juglandaceae</xref> family, particularly the black walnut, used as a colouring agent.