The Collaborative International Dictionary of English v.0.48:

Cerium \Ce"ri*um\, n. [Named by Berzelius in 1803 from the
   asteroid Ceres, then just discovered (1801).] (Chem.)
   A rare metallic element, occurring in the minerals cerite,
   allanite, monazite, etc. Symbol Ce. Atomic weight 141.5. It
   resembles iron in color and luster, but is soft, and both
   malleable and ductile. It tarnishes readily in the air.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cerium
    n 1: a ductile grey metallic element of the lanthanide series;
         used in lighter flints; the most abundant of the rare-earth
         group [syn: cerium, Ce, atomic number 58]

The Elements (07Nov00):

cerium
Symbol: Ce
Atomic number: 58
Atomic weight: 140.12
Silvery metallic element, belongs to the lanthanoids. Four natural
isotopes exist, and fifteen radioactive isotopes have been identified.
Used in some rare-earth alloys. The oxidized form is used in the glass
industry. Discovered by Martin .H. Klaproth in 1803.