Osmium (Os) is a chemical element with atomic number 76. With a density of 22.53 g/cm3, it is the densest of all stable elements. It is also extremely rare, making up around 0.000000005% of the Earth's crust, or 50 ppt (parts per trillion).
There are several naturally occurring osmium isotopes, but 192Os (76 protons, 116 neutrons) is the most abundant one.
Osmium was originally used in light bulbs (the German light bulb maker Osram's name is a combination of osmium and wolfram) but tungsten replaced osmium since it is both cheaper and more stable.