- a slightly crystalline, water-soluble, poisonous compound, HgO, occurring as a coarse, orange-red powder (red mercuric oxide ) or as a fine, orange-yellow powder (yellow mercuric oxide ): used chiefly as a pigment in paints and as an antiseptic in pharmaceuticals.
- a soluble poisonous substance existing in red and yellow powdered forms: used as pigments. Formula: HgO
