a radioactive chemical element, one of the rare-earth elements, obtained from fission of uranium or neutron bombardment of neodymium and used in phosphorescent paint, as a power source, X-ray source, etc.: symbol, Pm; at. no., 61
See promethium in American Heritage Dictionary 4
(prə-mēˈthē-əm)
noun Symbol Pm
A radioactive rare-earth element prepared by fission of uranium or by neutron bombardment of neodymium, having 14 isotopes with mass numbers ranging from 141 to 154 and used as a source of beta rays. Atomic number 61; melting point 1,168°C; boiling point 2,460°C; valence 3. See Table at element.