1580s, in reference to light (literally and figuratively), from French irradiation, noun of action from past participle stem of Latin irradiare (see irradiate). Of X-rays, etc., from 1901.
Entries linking to irradiation
irradiate v.
c. 1600, "to cast beams of light upon," from Latin irradiatus, past participle of irradiare "shine forth, beam upon, illumine," from assimilated form of in- "into, in" (from PIE root *en "in") + radiare "to shine" (see radiate (v.)). Meaning "expose to radiation other than light" (originally X-rays) is from 1901. Related: Irradiated; irradiating.