phi-phenomenon. n, pl-na (-nə). 1. (Psychology) the illusion that when two lights are rapidly turned on and off in succession something appears to move backwards and forwards between them while the lights stay stationary.hi-phenomenon. n, pl-na (-nə). 1. (Psychology) the illusion that when two lights are rapidly turned on and off in succession something appears to move backwards and forwards between them while the lights stay stationary.