1. A naturally radioactive, silvery, metallic transuranic element, occurring in uranium ores and produced artificially by neutron bombardment of uranium. its longest-lived isotope is Pu 244 with a half-life of 76 million years. it is a radiological poison, specifically absorbed by bone marrow, and is used, especially the highly fissionable isotope Pu 239, as a reactor fuel and in nuclear weapons. atomic number 94; melting point 640°C; boiling point 3,235°C; specific gravity 19.84; valence 3, 4, 5, 6. see table at element.

1. the transuranic chemical element with atomic number 94 and symbol Pu.

1. a solid silvery grey radioactive transuranic element whose atoms can be split when bombarded with neutrons; found in minute quantities in uranium ores but is usually synthesized in nuclear reactors; 13 isotopes are known with the most important being plutonium 239