[1] : The Collaborative International Dictionary of English v.0.48

actinium \actin"ium\, 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

[2] : WordNet (r) 2.0

actinium
     n : a radioactive element of the actinide series; found in
         uranium ores syn: ac, atomic number 89
see also:
ac atomic number 89 

[3] : Elements database 20001107

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.

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Actinium, definition of term: Actinium
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