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]

The Elements (07Nov00):

actinium
Symbol: Ac
Atomic number: 89
Atomic weight: (227)
Silvery radioactive metallic element, belongs to group 3 of the periodic
table. The most stable isotope, Ac-227, has a half-life of 217 years.
Ac-228 (half-life of 6.13 hours) also occurs in nature. There are 22
other
artificial isotopes, all radioactive and having very short half-lives.
Chemistry similar to lanthanum. Used as a source of alpha particles.
Discovered by A. Debierne in 1899.