1. Physics.
    1. the process in which energy is emitted as particles or waves.
    2. the complete process in which energy is emitted by one body, transmitted through an intervening medium or space, and absorbed by another body.
    3. the energy transferred by these processes.
  2. the act or process of radiating.
  3. something that is radiated.
  4. radial arrangement of parts.


  1. physics
    1. the emission or transfer of radiant energy as particles, electromagnetic waves, sound, etc
    2. the particles, etc, emitted, esp the particles and gamma rays emitted in nuclear decay
  2. Also called: radiation therapy med treatment using a radioactive substance
  3. anatomy a group of nerve fibres that diverge from their common source
  4. See adaptive radiation
  5. the act, state, or process of radiating or being radiated
  6. surveying the fixing of points around a central plane table by using an alidade and measuring tape

mid-15c., “act or process of radiating,” from Middle French radiation and directly from Latin radiationem (nominative radiatio) “a shining, radiation,” noun of action from past participle stem of radiare “to beam, shine, gleam; make beaming,” from radius “beam of light; spoke of a wheel” (see radius). Meaning “rays or beams emitted” is from 1560s. Meaning “divergence from a center” is 1650s.


  1. The act or condition of diverging in all directions from a center.
  2. The emission and propagation of energy in the form of rays or waves.
  3. The energy radiated or transmitted in the form of rays, waves, or particles.
  4. A stream of particles or electromagnetic waves that is emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.
  5. Radiotherapy.
  6. The radial arrangement of anatomical or histological parts.
  7. The spread of a group of organisms into new habitats.

    1. Streams of photons, electrons, small nuclei, or other particles. Radiation is given off by a wide variety of processes, such as thermal activity, nuclear reactions (as in fission), and by radioactive decay.
    2. The emission or movement of such particles through space or a medium, such as air. See Notes at conduction electromagnetic radiation.
  1. The use of such energy, especially x-rays, in medical diagnosis and treatment.

Energy sent out in the form of particles or waves. (See alpha radiation, beta radiation, blackbody, cosmic rays, electromagnetic radiation, fluorescence, gamma radiation, photon, and quanta.)

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