The Greek letter F is represented by the letter Phi (Φ). It is the 21st letter of the Greek alphabet and is pronounced 'FEE'. Phi is often used in mathematics, science, and engineering to represent a variety of constants, such as the golden ratio. It is also used in chemistry to represent the energy of a system's ground state. In addition, Phi is used in many other fields, such as philosophy, religion, and linguistics.