The Collaborative International Dictionary of English v.0.48:

Polonium \Po*lo"ni*um\ (p[-o]*l[=o]"n[i^]*[u^]m), n. [NL. So
   named after Poland, in L. form Polonia, one of the
   discoverers being a Pole.] (Chem.)
   A radioactive chemical element, discovered by M. and MMe.
   Curie in pitchblende, and originally called radium F. It
   has atomic number 84 and an atomic weight of 210. It is a
   very rare natural element, having an abundance in uranium
   ores only 0.2% that of radium. It is closely related
   chemically to bismuth. It emits only alpha rays, and has a
   half-life of 138 days. It is thus more unstable than radium,
   and a milligram of polonium emits as many alpha particles as
   5 grams of radium. Twenty-seven isotopes are known, with
   atomic masses from 192 to 218. At present a more practical
   method of preparation than isolation from ores is the
   preparation by neutron bombardment of bismuth in a nuclear
   reactor, and it may be obtained commercially by users having
   an appropriate permit. --[HBCF61]
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

polonium
    n 1: a radioactive metallic element that is similar to tellurium
         and bismuth; occurs in uranium ores but can be produced by
         bombarding bismuth with neutrons in a nuclear reactor [syn:
         polonium, Po, atomic number 84]

The Elements (07Nov00):

polonium
Symbol: Po
Atomic number: 84
Atomic weight: (210)
Rare radioactive metallic element, belongs to group 16 of the periodic
table. Over 30 known isotopes exist, the most of all elements. Po-209
has
a half-life of 103 years. Possible uses in heating spacecraft.
Discovered
by Marie Curie in 1898 in a sample of pitchblende.