The Collaborative International Dictionary of English v.0.48:

Praseodymium \Pra`se*o*dym"i*um\, n. [Praseo- + didymium.]
   (Chem.)
   An elementary substance, one of the constituents of didymium;
   -- so called from the green color of its salts. Symbol Ps.
   Atomic weight 143.6.
   [1913 Webster]

WordNet (r) 3.0 (2006):

praseodymium
    n 1: a soft yellowish-white trivalent metallic element of the
         rare earth group; can be recovered from bastnasite or
         monazite by an ion-exchange process [syn: praseodymium,
         Pr, atomic number 59]