1. (medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance

Synonym: radiotherapyradiation therapyradiationactinotherapy

2. (Pavolvian conditioning) the elicitation of a conditioned response by stimulation similar but not identical to the original stimulus

3. the apparent enlargement of a bright object when viewed against a dark background

4. (physiology) the spread of sensory neural impulses in the cortex

5. a column of light (as from a beacon)

Synonym: beambeam of lightlight beamrayray of lightshaftshaft of light

6. the condition of being exposed to radiation