1580s, from French irradiation, from Latin *irradiationem, noun of action from past participle stem of irradiare (see irradiate). Originally of light (literally and figuratively); of X-rays, etc., from 1901.
irradiation ir·ra·di·a·tion (ĭ-rā'dē-ā'shən)
Exposure or subjection to the action of radiation for diagnostic or therapeutic purposes.
Medical treatment by exposure to radiation.
The spread of a nervous impulse beyond the usual path of conduction.