The Collaborative International Dictionary of English v.0.48:

osmium \os"mi*um\ ([o^]z"m[i^]*[u^]m), n. [Gr. 'osmh` a smell,
   odor, akin to 'o`zein to smell. So named in allusion to the
   strong chlorinelike odor of osmic tetroxide. See Odor.]
   (Chem.)
   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 +PJC]

WordNet (r) 3.0 (2006):

osmium
    n 1: a hard brittle blue-grey or blue-black metallic element
         that is one of the platinum metals; the heaviest metal
         known [syn: osmium, Os, atomic number 76]

The Elements (07Nov00):

osmium
Symbol: Os
Atomic number: 76
Atomic weight: 190.2
Hard blue-white metallic transition element. Found with platinum and
used
in some alloys with platinum and iridium.