The symbol "Φ" (Phi) is the 21st letter of the Greek alphabet. In mathematics and science, it often represents the golden ratio, an important irrational number approximately equal to 1.618. The golden ratio appears in various natural phenomena, art, and architecture, symbolizing beauty and harmony. In addition to its mathematical significance, "Φ" is used in various fields such as physics, where it can denote magnetic flux, and in philosophy, where it may represent concepts related to form and essence. The letter is also associated with the Phi phenomenon in psychology, which describes the perception of motion in stationary images.