The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

actinium
    n 1: a radioactive element of the actinide series; found in
         uranium ores [syn: actinium, Ac, atomic number 89]