Radiation is energy that travels in waves or particles through space or a material, such as light, X-rays, or radio waves. It can be useful, like in medicine and communication, but some kinds can be harmful in large amounts.
It comes from Latin “radiatio,” from “radiare” meaning “to shine, emit rays,” and “radius” meaning “ray” or “spoke.” The modern scientific sense developed in the 1800s and 1900s with the study of light, heat, and radioactivity.
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