The Collaborative International Dictionary of English v.0.48:

iridium \i*rid"i*um\ ([-i]*r[i^]d"[i^]*[u^]m), n. [NL., fr. L.
   iris, iridis, the rainbow. So called from the iridescence of
   some of its solutions. See Iris.] (Chem.)
   A rare metallic element of atomic number 77, of the same
   group as platinum, which it much resembles, being
   silver-white and indifferent to most corrosive agents, but
   harder, more brittle, and with a higher melting temperature
   (2410[deg] C, versus platinum 1772[deg] C). With the
   exception of osmium, it is the heaviest substance known, its
   specific gravity being 22.4. Symbol Ir. Atomic weight 192.22.
   [1913 Webster +PJC]

   Note: Iridium usually occurs as a native alloy with osmium
         (iridosmine or osmiridium), which may occur alone or
         with platinum. Iridium, as an alloy with platinum, is
         used in bushing the vents of heavy ordnance. It is also
         used for the points of gold pens, and in a finely
         powdered condition (iridium black), for painting
         porcelain black.
         [1913 Webster]

WordNet (r) 3.0 (2006):

iridium
    n 1: a heavy brittle metallic element of the platinum group;
         used in alloys; occurs in natural alloys with platinum or
         osmium [syn: iridium, Ir, atomic number 77]

The Elements (07Nov00):

iridium
Symbol: Ir
Atomic number: 77
Atomic weight: 192.217
Very hard and brittle, silvery metallic transition element. It has a
yellowish cast to it. Salts of iridium are highly colored. It is the
most corrosion resistant metal known, not attacked by any acid, but is
attacked by molten salts. There are two natural isotopes of iridium, and
4 radioisotopes, the most stable being Ir-192 with a half-life of 73.83
days. Ir-192 decays into platinum, while the other radioisotopes decay
into osmium. Iridium is used in high temperature apparatus, electrical
contacts, and as a hardening agent for platinum. Discovered in 1803 by
Smithson Tennant in England. The name comes from the Greek word iris,
which
means rainbow. Iridium metal is generally non-toxic due to its relative
unreactivity, but iridium compounds should be considered highly toxic.