The crossword solution for the term 'LETTER AFTER 'X'' is 'PSI'. The letter 'PSI' is the 23rd letter of the Greek alphabet. It is also used in physics and chemistry to represent the wave function in quantum mechanics. In mathematics, it is used to represent the polygamma function. In astronomy, it is used to represent the surface density of a star or planet. In computing, it is used to represent the symbol for the programming language Python.