The term 'penultimate letter' refers to the second-to-last letter of a word. In this crossword, the solutions for the term are PSI and WYE, both of which are three-letter words. PSI is the 23rd letter of the Greek alphabet and is often used in physics and chemistry to represent the wave function in quantum mechanics. WYE is a letter in the English alphabet and is also used as a noun to refer to a Y-shaped configuration or junction.