noun
: a dark brown or gray-black insoluble compound MnO 2 found in nature as pyrolusite, made synthetically (as by decomposition of manganous nitrate or by chemical or electrolytic precipitation), and used chiefly as an oxidizing agent and a depolarizer in dry cells, in glassmaking and ceramics, as a pigment, and as a starting material for other manganese compounds (as permanganates and driers for varnishes and paints)