A rare metallic element of the platinum group with atomic number 76.  It is found native as an alloy in platinum ore, and in iridosmine.  It is a hard, infusible, bluish or grayish white metal, and the heaviest substance known.  Its tetroxide is used in histological experiments to stain tissues. Symbol 
Os.  Atomic weight 190.2.  Specific gravity 22.477.		                                        		                                         [
1913 Webster]