plutonium: 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
chemical element: any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter
circumflex: a diacritical mark (^) placed above a vowel in some languages to indicate a special phonetic quality