noun
: a slightly water-soluble crystalline poisonous compound HgO known in two forms (1) a yellow finely divided powder obtained usually by precipitation from solutions of mercury chloride (sense b) and sodium hydroxide and used chiefly in medicine (as in antiseptic ointments), in antifouling paints, and in making other mercury compounds (2) a bright red coarse powder obtained by precipitation from hot solutions or by heating mercurous nitrate and used similarly to the yellow form and also in dry cells — called also mercury(II) oxide