means Energy traveling outward through space or a medium as waves or particles — from sunlight and radio signals to the more dangerous emissions of nuclear decay.

from From the Latin 'radius,' meaning a ray, spoke, or rod — the same root that gives us 'ray' and 'radius.' The verb 'radiare,' to emit beams, gave English 'radiate,' and 'radiation' followed as the noun for that spreading-out of rays. Long before anyone knew about gamma rays or X-rays, the word simply described light fanning out like spokes from a wheel's hub; the menacing modern sense came only after physicists found energies the eye couldn't see.