A blurry halo effect that appears around bright areas in photographs, caused by light bouncing off the back of the film.
From 'hale' (halo) + '-ation' (suffix forming nouns). The photography term emerged in the early 1900s when film chemistry became precise enough to describe this specific optical defect.
Photographers used to spend money on special anti-halation coatings on film—now digital cameras avoid this problem entirely, but it's why old photos sometimes have that dreamy glow around streetlights!
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