The Collaborative International Dictionary of English v.0.48:

Selenium \Se*le"ni*um\, n. [NL., from Gr. selh`nh the moon. So
   called because of its chemical analogy to tellurium (from L.
   tellus the earth), being, as it were, a companion to it.]
   (Chem.)
   A nonmetallic element of the sulphur group of atomic number
   34, analogous to sulphur in its compounds. It is found in
   small quantities with sulphur and some sulphur ores, and
   obtained in the free state as a dark reddish powder or
   crystalline mass, or as a dark metallic-looking substance. It
   exhibits under the action of light a remarkable variation in
   electric conductivity, and is used in certain electric
   apparatus. Symbol Se. Atomic weight 78.96.
   [1913 Webster]

WordNet (r) 3.0 (2006):

selenium
    n 1: a toxic nonmetallic element related to sulfur and
         tellurium; occurs in several allotropic forms; a stable
         grey metallike allotrope conducts electricity better in the
         light than in the dark and is used in photocells; occurs in
         sulfide ores (as pyrite) [syn: selenium, Se, atomic
         number 34]