Archaic
term once used to denote all
radon isotopes collectively, when the
term radon was restricted to the
isotope radon-222, the naturally occurring
intermediate of the uranium-238
radioactive series; so called because
original names for radon-219, radon-220, and radon-222 were, respectively, "actinium emanation," "thorium emanation," and "radium emanation."
Origin: L. Emano, to
flow out + -on