osmium tetroxide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A volatile poisonous pale yellow solid, OsO₄, with a pungent smell, used as an oxidizing agent and in solution to stain and fix biological material, especially lipids.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry 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.