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]

The Elements (07Nov00):

thallium
Symbol: Tl
Atomic number: 81
Atomic weight: 204.3833
Pure, unreacted thallium appears silvery-white and exhibits a metallic
lustre. Upon reacting with air, it begins to turn bluish-grey and looks
like lead. It is very malleable, and can be cut with a knife. There are
two stable isotopes, and four radioisotopes, Tl-204 being the most
stable
with a half-life of 3.78 years. Thallium sulphate was used as a
rodenticide.
Thallium sulphine's conductivity changes with exposure to infrared
light,
this gives it a use in infrared detectors. Discovered by Sir William
Crookes
via spectroscopy. Its name comes from the Greek word thallos, which
means
green twig. Thallium and its compounds are toxic and can cause cancer.