The Collaborative International Dictionary of English v.0.48:

Thallium \Thal"li*um\, n. [NL., fr. Gr. ? young or green shoot
   or branch, twig. So called from a characteristic bright green
   line in its spectrum.] (Chem.)
   A rare metallic element of the aluminium group found in some
   minerals, as certain pyrites, and also in the lead-chamber
   deposit in the manufacture of sulphuric acid. It is isolated
   as a heavy, soft, bluish white metal, easily oxidized in
   moist air, but preserved by keeping under water. Symbol Tl.
   Atomic weight 203.7.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thallium
    n 1: a soft grey malleable metallic element that resembles tin
         but discolors on exposure to air; it is highly toxic and is
         used in rodent and insect poisons; occurs in zinc blende
         and some iron ores [syn: thallium, Tl, atomic number
         81]