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1. The diagnostic or therapeutic application of x-ray photons, nuclear particles, high-speed electrons, ultraviolet rays, or other forms of radiation to a patient.
2. The application of a form of radiation to an object or substance to give it therapeutic value or increase that which it already has.
3. A phenomenon in which a bright object on a dark background appears larger than a dark object of the same size on a bright background.
4. The spreading in all directions from a common center, e.g., nerve impulses, the sensation of pain.

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