Source : Webster's Revised Unabridged Dictionary (1913)

Selenium \Se*le"ni*um\, n. [NL., from Gr. ??? 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, and 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 78.9.

Source : WordNet®

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