ˈrāˌdän noun
( -s )
Etymology: New Latin, from rad- (from radium ) + -on
1. : a heavy radioactive gaseous element of the group of inert gases formed by disintegration of radium and used similarly to radium in medicine — symbol Rn ; called also radium emanation ; see element table, uranium series
2. : an isotope of radon: as
a. : actinon
b. : thoron