Definition from Wiktionary, the free dictionary
- Rhymes: -eɪʃən
- An act of irradiating, or state of being irradiated.
- (obsolete) illumination; irradiance; brilliance.
- (obsolete) figurative: mental light or illumination.
- (obsolete) 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, especially when a little out of focus.
- (uncountable) a process of sterilisation whereby radiation is passed through a bag containing food, utensils, etc., to sterilise the contents.
Derived terms 
obsolete: illumination; irradiance; brilliance
obsolete, figurative: mental light or illumination
obsolete: the apparent enlargement of a bright object seen upon a dark ground