Meaning of circle of confusion

cir'cle of confu'sion

Pronunciation: [key]
— Photog. Photog.
  1. a circular spot on a film, resulting from the degree to which a pencil of light reflected from the field of view is focused in front of or behind the film, or from aberration of the lens, or from both.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.