Ir*ra`di*a"tion (?), n. [Cf. F. irradiation.]
1.
Act of irradiating, or state of being irradiated.
2.
Illumination; irradiance; brilliancy.
Sir W. Scott.
3.
Fig.: Mental light or illumination.
Sir M. Hale.
4. Opt.
The apparent enlargement of a bright object seen upon a dark ground, due to the fact that the portions of the retina around the image are stimulated by the intense light; as when a dark spot on a white ground appears smaller, or a white spot on a dark ground larger, than it really is, esp. when a little out of focus.
© Webster 1913.