The letter phi () is the 21st letter of the Greek alphabet. It is a voiceless bilabial fricative, which means that it is produced by forcing air through the lips without vibration. The sound of phi is similar to the sound of the letter “f” in English.
Phi is used in a variety of words, including “philosophy,” “physics,” and “phonetics.” It is also used in the Greek numeral system, where it represents the number 500.