Radiation
Etymology
From Latin "radiare" (to emit rays), from "radius" (a ray, or the spoke of a wheel). The word covers everything that spreads out from a source like spokes from a hub.
noun · /ray-dee-AY-shun/
Energy that travels outward through space as waves or particles, from harmless radio and light to the ionizing kind that can damage cells.
What it names
A single family that spans the friendly (sunlight, wifi) and the dangerous (X-rays, nuclear decay), so “radiation” alone never tells you whether to worry.
Example
A banana emits a little radiation, a dental X-ray a bit more, and a nuclear meltdown vastly more — the word alone hides the enormous range.
Related idea
Not all radiation is radioactivity: radioactivity is the specific decay of unstable atomic nuclei, whereas radiation includes visible light and radio waves that carry no such hazard.