1. A discredited theory explaining the perception of motion pictures which assumes that an impression of the visual field (known as an after-image) either remains on the retina of the eye or is stored by the brain for approximately one twenty-fifth of a second before it decays and is supplanted by the next impression: see also apparent motion; flicker fusion; iconic memory; compare beta movement; phi phenomenon.

2. A smear effect that is the basis of the impact of firework displays and is produced by the dark-adapted eye, which compensates for low light levels by retaining light information over a short period of time. In conditions of bright ambient illumination this effect is lost.

http://www.grand-illusions.com/articles/persistence_of_vision/ Persistence of vision