Radioactivity
Etymology
From "radio-" (ray, from Latin "radius") plus "activity." Marie Curie coined the term around 1898 for the ray-emitting behavior she and Pierre studied in uranium and the new elements they isolated.
noun · /ray-dee-oh-ak-TIV-ih-tee/
The spontaneous breakdown of unstable atomic nuclei, which release energy and particles as they decay.
What it names
The fact that some atoms are inherently unstable and fall apart on their own schedule, powering both nuclear reactors and the clocks that date ancient rocks.
Example
The radioactivity of the buried fuel will take thousands of years to fade to safe levels.
Related idea
Radioactivity emits radiation, but the two are not the same: radiation is the broad family of energy in transit, while radioactivity is specifically the nuclear decay that produces the dangerous, ionizing kind.