People have been trained to be scared of the word “radiation”. But all it means is that something spreads outwards from a source. Sound counts as radiation. So do ripples on a pond, or earthquake waves in solid rock. If you use the term broadly enough, the shrapnel that blasts outward from a grenade can be considered as a form of radiation. And, of course, light is a form of radiation.
What people need to be legitimately scared of is the narrower category of ionizing radiation. That’s the nasty stuff that comes out of radioisotopes, nuclear reactions, and x-ray machines. It’s bad for ya because it destroys protein and DNA molecules inside your cells. Anything that’s capable of ionizing an atom is also capable of breaking apart an organic molecule in the process, and if that molecule is inside you, damaging enough of them in this way will give you radiation sickness, cancer, three-headed children, and so on.
The hot particles which spit out of radioisotopes, and which are generated in tremendous floods by nuclear reactors and bombs, are ionizing radiation. They aren’t waves, like sound or light (at least, no more than any solid object is wavelike) — they’re the subatomic equivalent of grenade shrapnel, consisting of solid pieces flung through the air. But the way that they shine straight out in all directions is such that the word “radiation” has never gone out of fashion for describing them.
X-rays and gamma rays are also ionizing radiation — especially the latter, which tend to be produced in nuclear reactions and accompany the other emissions of radioisotopes. But unlike the alpha and beta and other hot particles, these are electromagnetic waves. They are, essentially, light.
The heat on your skin from standing in front of a fire is also a form of light (infrared), but it is not ionizing. The only way it’ll ever disrupt organic molecules is by cooking them, and it’s far less effective for that purpose than, say, contact with hot water. But how can light be both ionizing and non-ionizing?