The Collaborative International Dictionary of English v.0.48:

Platinum \Plat"i*num\, n. [NL., fr. Sp. platina, from plata
   silver, LL. plata a thin plate of metal. See Plate, and cf.
   Platina.] (Chem.)
   A metallic element of atomic number 78, one of the noble
   metals, classed with silver and gold as a precious metal,
   occurring native or alloyed with other metals and also as the
   platinum arsenide (sperrylite). It is a heavy tin-white metal
   which is ductile and malleable, but very infusible (melting
   point 1772[deg] C), and characterized by its resistance to
   strong chemical reagents. It is used for crucibles in
   laboratory operations, as a catalyst, in jewelry, for stills
   for sulphuric acid, rarely for coin, and in the form of foil
   and wire for many purposes. Specific gravity 21.5. Atomic
   weight 195.1. Symbol Pt. Formerly called platina.
   [1913 Webster +PJC]

   Platinum black (Chem.), a soft, dull black powder,
      consisting of finely divided metallic platinum obtained by
      reduction and precipitation from its solutions. It absorbs
      oxygen to a high degree, and is employed as an oxidizer.


   Platinum lamp (Elec.), a kind of incandescent lamp of which
      the luminous medium is platinum. See under Incandescent.


   Platinum metals (Chem.), the group of metallic elements
      which in their chemical and physical properties resemble
      platinum. These consist of the light platinum group, viz.,
      rhodium, ruthenium, and palladium, whose specific
      gravities are about 12; and the heavy platinum group,
      viz., osmium, iridium, and platinum, whose specific
      gravities are over 21.

   Platinum sponge (Chem.), metallic platinum in a gray,
      porous, spongy form, obtained by reducing the double
      chloride of platinum and ammonium. It absorbs oxygen,
      hydrogen, and certain other gases, to a high degree, and
      is employed as an agent in oxidizing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

platinum black
    n 1: a fine black powder of platinum; used as a catalyst in
         chemical reactions