The term 'RAPID PLUNGE' is a combination of two words, 'RAPID' and 'PLUNGE'. The word 'RAPID' means to move quickly or to happen quickly, while the word 'PLUNGE' means to dive or to fall suddenly. Together, the term 'RAPID PLUNGE' can be used to describe a sudden, RAPID descent or drop. In this case, the crossword solution for the term 'RAPID PLUNGE' is 'TAILSPIN', which is a type of RAPID descent or dive that is often seen in aircrafts. A tail spin is a RAPID, uncontrolled descent in which the aircraft's nose drops and the aircraft rotates around its vertical axis.