ˈthōrˌän, ˈthȯˌrän noun
( -s )
Etymology: New Latin, from thor- (from thorium ) + -on
: a heavy radioactive gaseous isotope of the group of inert gases that is isotopic with radon and actinon, is formed by disintegration of thorium, emits alpha rays, and has a half-life of less than a minute (mass number 220) — called also thorium emanation ; see thorium series