n. [ Pyro- + Gr. &unr_; to loose, or &unr_; a loosing. ] (Min.) Manganese dioxide, a mineral of an iron-black or dark steel-gray color and metallic luster, usually soft. Pyrolusite parts with its oxygen at a red heat, and is extensively used in discharging the brown and green tints of glass (whence its name). [ 1913 Webster ]