The Collaborative International Dictionary of English v.0.48:

Tellurium \Tel*lu"ri*um\, n. [NL., from L. tellus, -uris, the
   earth.] (Chem.)
   A rare nonmetallic element, analogous to sulphur and
   selenium, occasionally found native as a substance of a
   silver-white metallic luster, but usually combined with
   metals, as with gold and silver in the mineral sylvanite,
   with mercury in Coloradoite, etc. Symbol Te. Atomic weight
   125.2.
   [1913 Webster]

   Graphic tellurium. (Min.) See Sylvanite.

   Tellurium glance (Min.), nagyagite; -- called also black
      tellurium.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tellurium
    n 1: a brittle silver-white metalloid element that is related to
         selenium and sulfur; it is used in alloys and as a
         semiconductor; occurs mainly as tellurides in ores of
         copper and nickel and silver and gold [syn: tellurium,
         Te, atomic number 52]