osmium tetroxide(Noun)

An oxide of the metal osmium, OsO; it is very poisonous, has a pungent smell and can cause blindness; it is a powerful oxidizing agent, and is used to stain biological materials.

Osmium Tetroxide

(T-4)-Osmium oxide (OsO4). A highly toxic and volatile oxide of osmium used in industry as an oxidizing agent. It is also used as a histological fixative and stain and as a synovectomy agent in arthritic joints. Its vapor can cause eye, skin, and lung damage.