What is the 21st letter of the greek alphabet?

The 21st letter of the Greek alphabet is "phi" (Φ, φ). It is a consonant and is pronounced as "f" in English. In Greek numerals, it has a value of 500. The letter phi is often used in mathematics as a symbol for the golden ratio, which has many applications in geometry, art, and design. It is also used in physics and engineering to represent electric flux. In addition, "phi" is commonly used as an abbreviation for "philosophy" or "philosopher."