Main Entry:

plu·to·ni·um

Pronunciation:

\plü-ˈtō-nē-əm\

Function:

noun

Etymology:

New Latin, from Pluton-, Pluto, the planet Pluto

Date:

1942

: a radioactive metallic element similar chemically to uranium that is formed as the isotope 239 by decay of neptunium and found in minute quantities in pitchblende, that undergoes slow disintegration with the emission of an alpha particle to form uranium 235, and that is fissionable with slow neutrons to yield atomic energy — see element table