Definition of protactinium noun from the Oxford Advanced Learner's Dictionary

 

protactinium

 noun
noun
BrE BrE//ˌprəʊtækˈtɪniəm//
 
; NAmE NAmE//ˌproʊtækˈtɪniəm//
 
[uncountable](symbol Pa)
 
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a chemical element. Protactinium is a radioactive metal found naturally when uranium decays. Word Origin early 20th cent.: from proto- ‘original, earlier’ + actinium, so named because one of its isotopes decays to form actinium.
See the Oxford Advanced American Dictionary entry: protactinium