phi

noun3

1. •
   Φ, the 21st letter of the Euclidean and modern Greek alphabet, usually romanized as "ph".
2. •
   The golden ratio.
3. •
   A visual illusion whereby a sequential pattern of lights produces a false sense of motion.

   • 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

noun1

1. •
   Initialism of protected health information.