The penultimate Greek letter is PSI. In the Greek alphabet, PSI is the 23rd letter and is represented by the symbol Ψ. It is commonly used in various fields such as physics, mathematics, and engineering to represent concepts such as wave functions, electric flux, and stress. In addition, PSI is also used in the field of psychology to represent the concept of the psyche or the human mind. Overall, PSI is an important letter in the Greek alphabet and has significant applications in various fields of study.