promethium
noun
pro·me·thi·um prə-ˈmē-thē-əm 
: a radioactive metallic element of the rare-earth group obtained as a fission product of uranium or from neutron-irradiated neodymium see Chemical Elements Table
Example Sentences
Recent Examples on the Web One promethium isotope, with eighty-eight neutrons, has a half-life of a few days; the other, with eighty-six neutrons, has a half-life of a few years. 
Neima Jahromi, The New Yorker, 27 Dec. 2019
Neima Jahromi, The New Yorker, 27 Dec. 2019 Word History
Etymology
New Latin, from Latin Prometheus
First Known Use
1948, in the meaning defined above
Medical Definition
promethium
noun
pro·me·thi·um prə-ˈmē-thē-əm
: a radioactive metallic element of the rare-earth group obtained as a fission product of uranium or from neutron-irradiated neodymium —symbol Pm see Chemical Elements Table