WordNet (r) 3.0 (2006):

plutonium
    n 1: a solid silvery grey radioactive transuranic element whose
         atoms can be split when bombarded with neutrons; found in
         minute quantities in uranium ores but is usually
         synthesized in nuclear reactors; 13 isotopes are known with
         the most important being plutonium 239 [syn: plutonium,
         Pu, atomic number 94]

The Elements (07Nov00):

plutonium
Symbol: Pu
Atomic number: 94
Atomic weight: (242)
Dense silvery radioactive metallic transuranic element, belongs to the
actinoids. Pu-244 is the most stable isotope with a half-life of
7.6*10^7
years. Thirteen isotopes are known. Pu-239 is the most important, it
undergoes nuclear fission with slow neutrons and is hence important to
nuclear weapons and reactors. Plutonium production is monitored down to
the gram to prevent military misuse. First produced by Gleen T. Seaborg,
Edwin M. McMillan, J.W. Kennedy and A.C. Wahl in 1940.