Phi is the 21st letter of the Greek alphabet. It is pronounced as 'fee' or 'fie' by the Greeks. It is also occasionally pronounced as 'fai'. The mathematical value of this letter is 500 (φʹ) or 500,000 (͵φ). Φ φ is the symbol of Phi. It is also known as the golden number. Its shape is considered aesthetically pleasing in Western cultures and continuously used in art and design as it suggests a natural balance between symmetry and asymmetry. This letter is used to denote several things. It is used in geometry as well. There is something known as the phi phenomenon which refers to the apparent motion that results due to an orderly sequence of stimuli without any actual motion visible to the eye.