Source : Webster's Revised Unabridged Dictionary (1913)

Actinium \Ac*tin"i*um\, n. [Gr. ?, ?, ray.] (Chem.)
   A supposed metal, said by Phipson to be contained in
   commercial zinc; -- so called because certain of its
   compounds are darkened by exposure to light.

Source : WordNet®

actinium
     n : a radioactive element of the actinide series; found in
         uranium ores [syn: {Ac}, {atomic number 89}]