Radiation is energy transmitted in waves or a stream of particles. The first thing you think of as radiation is probably X-rays, but what cooks your food in the microwave oven is also radiation.

Radiation has some important medical uses, especially in the treatment of cancer, as "radiation therapy." It’s also associated with such horrors as "radiation sickness" — observed in people exposed to high levels of radiation, as in the aftermath of the 1986 explosion at the Chernobyl nuclear reactor, in the Ukraine. You can also use the word radiation, in more benign contexts, to describe something spreading out from a central point, as the radiation of warmth from your fireplace.

Definitions of radiation
  1. noun
    the act of spreading outward from a central source
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    the act (or an instance) of subjecting a body or substance to the impact of high-energy particles (as electrons or alpha rays)
    type of:
    emanation, emission
    the act of emitting; causing to flow forth
  2. noun
    a radial arrangement of nerve fibers connecting different parts of the brain
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    type of:
    neural structure
    a structure that is part of the nervous system
  3. noun
    the spread of a group of organisms into new habitats
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    adaptive radiation
    the development of many different forms from an originally homogeneous group of organisms as they fill different ecological niches
    type of:
    spread, spreading
    process or result of distributing or extending over a wide expanse of space
    action, activity, natural action, natural process
    a process existing in or produced by nature (rather than by the intent of human beings)
  4. noun
    energy that is radiated or transmitted in the form of rays or waves or particles
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    ionizing radiation
    high-energy radiation capable of producing ionization in substances through which it passes
    cosmic radiation
    radiation coming from outside the solar system
    electromagnetic radiation, electromagnetic wave, nonparticulate radiation
    radiation consisting of waves of energy associated with electric and magnetic fields resulting from the acceleration of an electric charge
    solar radiation
    radiation from the sun
    actinic radiation, actinic ray
    electromagnetic radiation that can produce photochemical reactions
    alpha radiation, alpha ray
    the radiation of alpha particles during radioactive decay
    beam, electron beam, ray
    a group of nearly parallel lines of electromagnetic radiation
    beta radiation, beta ray, electron radiation
    radiation of beta particles during radioactive decay
    black-body radiation, blackbody radiation
    the electromagnetic radiation that would be radiated from an ideal black body; the distribution of energy in the radiated spectrum of a black body depends only on temperature and is determined by Planck's radiation law
    CBR, CMB, CMBR, cosmic background radiation, cosmic microwave background, cosmic microwave background radiation
    (cosmology) the cooled remnant of the hot big bang that fills the entire universe and can be observed today with an average temperature of about 2.725 kelvin
    cosmic ray
    highly penetrating ionizing radiation of extraterrestrial origin; consisting chiefly of protons and alpha particles; collision with atmospheric particles results in rays and particles of many kinds
    Hertzian wave
    an electromagnetic wave generated by oscillations in an electric circuit
    gamma radiation, gamma ray
    electromagnetic radiation emitted during radioactive decay and having an extremely short wavelength
    a single frequency (or very narrow band) of radiation in a spectrum
    a short electromagnetic wave (longer than infrared but shorter than radio waves); used for radar and microwave ovens and for transmitting telephone, facsimile, video and data
    neutron radiation
    radiation of neutrons (as by a neutron bomb)
    radio emission, radio radiation, radio wave
    an electromagnetic wave with a wavelength between 0.5 cm to 30,000 m
    incident solar radiation
    flare, solar flare
    a sudden eruption of intense high-energy radiation from the sun's surface; associated with sunspots and radio interference
    solar prominence
    large eruptions of luminous hydrogen gas that rise thousands of kilometers above the chromosphere
    solar wind
    a stream of protons moving radially from the sun
    X ray, X-radiation, X-ray, roentgen ray, x-ray
    electromagnetic radiation of short wavelength produced when high-speed electrons strike a solid target
    type of:
    energy, free energy
    (physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs
  5. noun
    the spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay
    synonyms: radioactivity
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    corpuscular radiation, particulate radiation
    a stream of atomic or subatomic particles that may be charged positively (e.g. alpha particles) or negatively (e.g. beta particles) or not at all (e.g. neutrons)
    alpha radiation, alpha ray
    the radiation of alpha particles during radioactive decay
    beta radiation, beta ray, electron radiation
    radiation of beta particles during radioactive decay
    neutron radiation
    radiation of neutrons (as by a neutron bomb)
    type of:
    the release of electrons from parent atoms
  6. noun
    (medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance
    synonyms: actinotherapy, irradiation, radiation therapy, radiotherapy
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    the use of strong light to treat acne or hyperbilirubinemia of the newborn
    Curietherapy, radium therapy
    the use of radium in radiation therapy
    X-ray therapy
    the therapeutic use of X rays
    type of:
    (medicine) the act of caring for someone (as by medication or remedial training etc.)
  7. noun
    syndrome resulting from exposure to ionizing radiation (e.g., exposure to radioactive chemicals or to nuclear explosions); low doses cause diarrhea and nausea and vomiting and sometimes loss of hair; greater exposure can cause sterility and cataracts and some forms of cancer and other diseases; severe exposure can cause death within hours
    “he was suffering from radiation
    synonyms: radiation sickness, radiation syndrome
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    type of:
    a pattern of symptoms indicative of some disease
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