The Collaborative International Dictionary of English v.0.48:

Neodymium \Ne`o*dym"i*um\, n. [NL. Dee Neo-, and Didymium.]
   (Chem.)
   The chemical element of atomic number 60, one of the rare
   earth elements. Symbol Nd. Atomic weight 144.27.

   Note: It is a rare metallic element occurring in combination
         with cerium, lanthanum, and other rare metals, and
         forming amethyst-colored salts. It was originally
         thought to be part of a supposed new element didymium,
         obtained from cerite in 1843. Later this was resolved
         into two elements, neodymium and praesodymium by von
         Welsbach in 1885. It is chiefly trivalent. It is a
         faintly yellow metal.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

neodymium
    n 1: a yellow trivalent metallic element of the rare earth
         group; occurs in monazite and bastnasite in association
         with cerium and lanthanum and praseodymium [syn:
         neodymium, Nd, atomic number 60]