phi

Etymology

From Late Latin phi, from Ancient Greek φεῖ (pheî).

noun

1. Φ, the 21st letter of the Euclidean and modern Greek alphabet, usually romanized as "ph".
2. (mathematics) The golden ratio.
3. A visual illusion whereby a sequential pattern of lights produces a false sense of motion.
   […] tolerance (Frenkel-Brunswik 1949) for it showed that those persons with the strongest defences against internal conflict, i.e. obsessionals and conversion hysterics, were those least able to perceive the reality-conflicting phenomenon of phi. 1963, Psychology Through Experiment (George Humphrey, Jaroslav Antonio Deutsch), page 90
   The illusion of movement in a highway construction sign is an instance of the phi phenomenon, which is also at work in motion pictures and television. 2007, Wayne Weiten, Psychology: Themes and Variations: Themes And Variations, page 138
   Is it the phi phenomenon or the persistence of vision? Originally, the illusion of motion that movies excel at was thought to be the effect of the persistence of vision. 2013, Francis Glebas, The Animator's Eye: Composition and Design, page 66