The term 'Sonata POSTSCRIPT' is composed of two words: 'sonata' and 'POSTSCRIPT'. A sonata is a type of musical composition, usually for one or two instruments, that is typically in three or four movements. A POSTSCRIPT is an additional thought or comment added to the end of a letter or other written document. In this case, the crossword solution for the term is 'CODA', which is a concluding section of a piece of music, often used to bring the piece to a satisfying and conclusive end.