What is "osmium tetroxide"

n. (context chemistry English) An oxide of the metal osmium, OsO₄; 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 (also osmium(VIII) oxide) is the chemical compound with the formula OsO. The compound is noteworthy for its many uses, despite the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity OsO, which is yellow-brown in colour.