1.hopeanharmaa radioaktiivinen transuraaninen alkuaine (plutonium), jota tuotetaan yleensä ydinreaktoreissa
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