halation (həˈleɪ.∫ən). [irreg. f. HALO + -ATION.]

a. Photogr. The term used to denote the spreading of light beyond its proper boundary in the negative image upon the plate, producing local fog around the high lights, or those portions of the picture which are brighter than the rest of the image.

b. A similar effect in television.

Oxford English Dictionary