Osmium's atomic number is 76 and its symbol is Os. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element, with an experimentally measured (using x-ray crystallography) density of 22.65 g/cm3. The element's abundance is estimated to be about 0.6 parts per billion in the universe and is, therefore, the rarest precious metal.

The element is named after Greek word "osme" meaning "a smell", because of the ashy and smoky smell of the volatile compound it was first discovered on. Osmium's discovery was intertwined with the study of platinum, due to both being found in platinum ores.

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